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Research Techniques Made Simple: Experimental UVR Exposure

  • Paul O’Mahoney
    Affiliations
    Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom

    Division of Molecular and Clinical Medicine, School of Medicine, University of Dundee, Dundee, United Kingdom
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  • Victoria A. McGuire
    Affiliations
    Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom
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  • Robert S. Dawe
    Affiliations
    Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom

    Division of Molecular and Clinical Medicine, School of Medicine, University of Dundee, Dundee, United Kingdom
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  • Ewan Eadie
    Affiliations
    Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom
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  • Sally H. Ibbotson
    Correspondence
    Correspondence: Sally H. Ibbotson, Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom.
    Affiliations
    Photobiology Unit, NHS Tayside, Ninewells Hospital & Medical School, Dundee, United Kingdom

    Division of Molecular and Clinical Medicine, School of Medicine, University of Dundee, Dundee, United Kingdom
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      UVR exposure is a widely applied technique in clinical and preclinical studies. Such experimental conditions provide crucial information on the biological responses of skin and cell models, which may then be extrapolated and interpreted, for example, in the context of equivalent daylight exposures. It is therefore important to fully understand the characteristics of UVR and the principles behind correct and appropriate UVR exposure in experimental settings. In this Research Techniques Made Simple article, we discuss the relevant background information and the best practices for accurate, transparent, and reproducible experimentation and reporting of UVR exposure.

      Abbreviations:

      CoV (coefficient of variation), MED (minimal erythema dose)

      Introduction

      The effects of UVR are diverse and far reaching. Controlled UVR exposure is commonly required in both preclinical and clinical research settings to address questions about its biological effects on skin or surrogate skin models. Experimental studies of both natural daylight and artificial UVR exposure have underpinned many of the landmark discoveries in dermatological research and our understanding of physiological and pathological processes. For example, preclinical and clinical models of UVR exposure have profoundly increased our understanding of the wavelength dependency of photocarcinogenesis, the characteristics of UVR-induced immunosuppression and UVR-induced erythema. These experimental models of UVR exposure have also advanced our understanding of drug-induced photosensitivity and photoprotective agents and have informed our development of photodermatological practices, both photodiagnostic and phototherapeutic. For example, the pioneering work of
      • Parrish J.A.
      • Jaenicke K.F.
      Action spectrum for phototherapy of psoriasis.
      informed our understanding of the action spectrum for the clearance of psoriasis and in turn the development of narrowband UVB phototherapy, facilitating effective clearance of psoriasis while minimizing erythemal risk.

      Summary Points

      What information the assay or technique provides

      These measurement techniques provide a comprehensive characterization of UVR exposure in experimental and clinical settings. The information gives clarity and robustness and encourages the reproducibility of the exposures carried out.

      Limitations of the assay or technique

      The quality of measurements and information provided depends on the equipment available to the user. Simple measurements are inexpensive and still very useful; however, a full characterization of light sources may require more specialist tools. If these tools are unavailable, it is advised to contact other centers with photobiology clinical and/or laboratory expertise who may be able to assist and therefore produce a more complete understanding of the relevant UVR exposure.
      All too often, contrasting and conflicting data appear in the literature, which may arise owing to a lack of understanding of the characteristics of UVR and the principles of its interactions with in vivo or in vitro skin or equivalent models. Photophysics underpins all aspects of UVR exposure, and there are several ways of quantifying this radiation. It is critical that these are defined and understood (Table 1). Adhering to the following photophysics core principles will help ensure that UVR exposure is understood and correctly undertaken.
      Table 1Key Photophysics Parameters
      ParametersDescription
      Power (W)In this context, it relates to the total optical output of the light source in all directions. It is measured in Watts (W) and not normally useful to report with noncoherent UVR exposures.
      Irradiance (W/m2)Power falling on a surface per unit area (W/m2). It should always be reported alongside information on the spectral range to which it applies, for example, irradiance = 10 W/m2 (280–400 nm).
      Spectral irradiance (W/m2/nm)Irradiance per unit wavelength (W/m2/nm) is normally displayed in graphical format (Figure 1). It should ideally be displayed on both logarithmic and nonlogarithmic y-axes.
      Radiant exposure (J/m2)Often referred to as dose.

      Radiant exposure (J/m2) = Irradiance (W/m2) x Exposure time (s)
      ReciprocityWhen the biological effects depend on the radiant exposure only and not the way it was delivered, that is, the same biological effect from 10 J/m2 delivered as 10 W/m2 in 1 s or 1 W/m2 in 10 s. Reciprocity cannot always be assumed.
      WavelengthRefers to a specific wavelength of light per nm, for example, 350 nm.
      WavebandDefines a range of wavelengths, for example, 315–400 nm, although sometimes, it is written as the FWHM about a central wavelength, for example, 350 ± 10 nm.
      Abbreviation: FWHM, full-width half maximum.

      Photophysics

      Core principle 1: Safety

      It is key that risk assessment is performed before working with UVR and that control measures are used to prevent exceeding exposure limit values. Exposure limits for UVR were published by the
      International Commission on Non-Ionizing Radiation Protection
      ICNIRP guidelines on limits of exposure to ultraviolet radiation of wavelengths between 180 nm and 400 nm (incoherent optical radiation).
      , and each country should have incorporated these into their own legislation. In the hierarchy of control measures, engineering controls are the most effective protection measure (e.g., UVR enclosed within a box), followed by administrative controls (e.g., staff training and standard operating procedures). Personal protective equipment such as UV protective glasses is the least effective, and whereas their use may be required, it should not be relied on as the only risk control.

      Core principle 2: UV spectrum

      UVR (100–400 nm) can be subdivided: UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm), with UVA further subdivided into UVA1 (340–400 nm) and UVA2 (315–340 nm). It is worth noting that these ranges are commonly used but can vary depending on convention. UVC wavelengths are absorbed by the atmosphere and are therefore not relevant when considering solar exposures at the terrestrial level; however, there is a growing interest in using artificial UVC sources (∼222 nm) to disinfect surfaces, including human skin. UV sources are often referred to by their respective wavebands, for example, UVA lamp. Whereas this naming convention may be convenient, it is not descriptive enough and does not include enough detailed information to fully understand the characteristics of any experimental irradiation procedure performed with that source.
      The emission spectrum of a light source is essential, providing critical information on wavelength-dependent biological mechanisms and enabling action spectra to be defined (
      • Liebel F.
      • Kaur S.
      • Ruvolo E.
      • Kollias N.
      • Southall M.D.
      Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes.
      ). In many instances, the dose is the only quantity reported in studies; however, without the prerequisite information on UV spectrum and irradiance, the dose can be misleading.
      For example, a publication may report that a UVA lamp with a dose of 100 J/m2 caused erythema in the skin of 10 subjects and may then make a broad statement that UVA radiation causes erythema. However, without detail on the spectrum of the lamp, it is unclear which wavelengths of radiation caused the erythema. Take for example the UVA fluorescent lamp spectrum in Figure 1. If the spectrum is combined with the International Commission on Illumination erythema action spectrum (Figure 2), it is clear that the most effective wavelengths at causing erythema in this lamp are the UVB wavelengths that are emitted, some of which are not even visible in the original nonlogarithmic-plotted spectral irradiance.
      Figure thumbnail gr1
      Figure 1Fluorescent lamp (PL-L 36W/09/4P, Philips Lighting, Amsterdam, Netherlands) spectral irradiance on a non-Log. and log scale. Log., logarithmic.
      Figure thumbnail gr2
      Figure 2Demonstrates the importance of knowing the lamp spectrum over the full wavelength range and plotting the spectrum on a logarithmic scale. If only had been reported, it would have been missed that there are lower irradiance emissions in the UVB, which are extremely effective at causing erythema. The 298 nm radiation is 1,400 times more effective at causing erythema than 350 nm radiation.
      The principles described by this example apply not only to erythema but also to any other biological effect. Very low UVR emissions at a given wavelength may be the cause of the biological effect if they are more effective than higher UVR emissions at other wavelengths.

      Core principle 3: Calibration

      The key to the accurate measurement of light is the use of a calibrated detector (
      • Moseley H.
      • Allan D.
      • Amatiello H.
      • Coleman A.
      • Du Peloux Menagé H.
      • Edwards C.
      • et al.
      Guidelines on the measurement of ultraviolet radiation levels in ultraviolet phototherapy: report issued by the British Association of Dermatologists and British Photodermatology Group 2015.
      ), traceable to national standards (e.g., National Physical Laboratory). If using a broadband light detector to measure irradiance, it is crucial that this is done with the knowledge of the output spectrum of the light source and is calibrated to each light source measured. Although more costly, using a calibration laboratory that is International Organization for Standardization 17025 accredited for optical calibration will provide confidence that the calibration is appropriate for the task. Always describe the process you are undertaking to the calibration laboratory and ask whether they can provide an appropriate calibration.
      Example of ideal reporting of UVR exposure:
      • The skin was exposed to 100 J/m2 of UVR (280-400 nm) from a fluorescent lamp (PL-L 36W/09/4P, Philips Lighting, Amsterdam, Netherlands). A stable irradiance of 2 W/m2 was measured with a calibrated radiometer (ILT2400 meter, SEL033 detector, UVA filter, Teflon diffuser, calibrated on March 5, 2020). Exposure time was 50 seconds and the spectral irradiance of the lamp is shown in Figure 1.

      Core principle 4: Uniformity of illumination

      It is important to measure the uniformity of UVR distribution within the irradiation area (
      • Moseley H.
      • Allan D.
      • Amatiello H.
      • Coleman A.
      • Du Peloux Menagé H.
      • Edwards C.
      • et al.
      Guidelines on the measurement of ultraviolet radiation levels in ultraviolet phototherapy: report issued by the British Association of Dermatologists and British Photodermatology Group 2015.
      ). Different regions within an irradiation field, for example, a multiwell plate, may be subject to a higher or lower irradiance than others, resulting in different doses being delivered to different sites within the field. Uniformity may be measured at the irradiation field by measuring irradiance in multiple locations across the irradiation field (Figure 3), for example, the center of the plate and at the corners. Uniformity may then be characterized by observing the mean, median, SD, and coefficient of variation (CoV), which is the SD expressed as a percentage of the mean. The CoV should be kept as low as possible, and the difference between any two measurement values should not be more than 10%.
      Figure thumbnail gr3
      Figure 3An example of carrying out an assessment of uniformity of light distribution. CoV, coefficient of variation; Diff., difference; Max., maximum; Min., minimum.
      It can be difficult to improve the uniformity of light irradiation. Changing the distance between the light source and irradiation field will typically change the uniformity but will also change the irradiance. Caution should be exercised in both maintaining an appropriate irradiance for the study and a realistic irradiation time.

      Core principle 5: UVR source stability

      The output of a UVR source may not be stable and will typically decrease during its lifetime. It is important to characterize the source output stability before use. This can be done by performing measurements at set time periods (e.g., every 30 seconds) for the anticipated maximum exposure time of the experiment plus a warm-up period. It is common practice for a warm-up period to be defined as the length of time required for the lamp emission to become relatively stable for the exposure time of the experiment (e.g., less than a 5% change in measured irradiance). Once stable, a single irradiance measurement can be multiplied by the exposure time to determine the radiant exposure. With unstable lamps, it can also be possible to leave a detector in situ and measure the cumulative exposure. Such a process should be undertaken with caution, ensuring that the detector does not influence and is representative of the UVR incident on the experimental setup.

      Preclinical testing

      When investigating the effects of UVR in cell culture models in vitro, endpoints that can be considered as indicators of the biological effects of UVR include the induction of cell cycle arrest or apoptosis and quantification of DNA damage. UV-induced DNA damage can be assessed by measuring DNA comets or quantifying cyclobutane pyrimidine dimers and 6–4 photoproduct formation. These photoproducts can be detected using a variety of techniques, including molecular, chromatographic, and mass spectrometry strategies (
      • Figueroa-González G.
      • Pérez-Plasencia C.
      Strategies for the evaluation of DNA damage and repair mechanisms in cancer.
      ). Detection and quantification of these indicators of DNA damage can also be undertaken in animal models and human skin after UV irradiation.
      If investigating the effects of drug and light interactions in vitro, regulatory requirements state that any drug with a molar absorption coefficient of >1,000 l/mol/cm at any wavelength between 290 nm and 700 nm must undergo photosafety evaluation (
      Food and Drug Administration
      S10 Photosafety evaluation of pharmaceuticals - Guidance for industry.
      ), with the first step being the in vitro 3T3 Neutral Red Uptake Phototoxicity test (
      OECD
      OECD guideline for testing of chemicals “UV-VIS Absorption Spectra”.
      ). The initial assessment of phototoxic potential can also include photoreactivity assessments measuring the formation of type I (superoxide) and type II (singlet oxygen) ROSs after irradiation with UV-visible radiation (
      • Onoue S.
      • Igarashi N.
      • Yamada S.
      • Tsuda Y.
      High-throughput reactive oxygen species (ROS) assay: an enabling technology for screening the phototoxic potential of pharmaceutical substances.
      ). Whichever light source is used, it is important to measure both singlet oxygen and superoxide species to avoid the production of false negatives (
      • Ceridono M.
      • Tellner P.
      • Bauer D.
      • Barroso J.
      • Alépée N.
      • Corvi R.
      • et al.
      The 3T3 neutral red uptake phototoxicity test: practical experience and implications for phototoxicity testing--the report of an ECVAM-EFPIA workshop.
      ), especially as negative preclinical results indicate a low probability of phototoxicity and can support a decision not to undertake further photosafety evaluation in humans.
      In vitro UV irradiation in cell models or skin equivalents, whether involving a drug or other intervention, follows the same broad principles outlined previously alongside appropriate controls. This may include at least four study arms with each subject to the same variations in drug concentration and radiant exposure:
      • 1.
        unirradiated cells with no drug per intervention applied,
      • 2.
        irradiated cells with no drug per intervention applied,
      • 3.
        unirradiated cells with drug per intervention applied, and
      • 4.
        irradiated cells with drug per intervention applied.
      Preclinical studies in cell models may be difficult to interpret in terms of extrapolation to the human setting, and thus, it may be necessary to undertake UV irradiation studies in animal models. The same principles remain to fully understand the rationale behind the choice of irradiation source and the characteristics of UV exposure.

      Clinical phototesting

      Many clinical research studies require phototesting. There may be very different research questions posed, such as in drug phototoxicity studies, photoprotection studies, studies to understand the biological effects of UV and wavelength dependency. Once the study objectives are outlined, the key aspects of clinical phototesting are to decide on which UV wavelengths should be used: should it be a narrowband or broadband source; is it single exposures or iterative testing; which dose ranges and irradiances to use; are normal range values available?
      The endpoint in clinical phototesting is often a measurement of the minimal erythema dose (MED). We define this as the minimum dose of light that produces just perceptible reddening of the skin at 24 hours after irradiation (
      • Ouinn A.
      • Diffey B.
      • Craig P.
      • Farr P.
      Definition of the minimal erythemal dose used for diagnostic phototesting.
      ). MED gives a measure of the threshold erythemal sensitivity of the skin after exposure to any given UVR light source and will be influenced by the emission spectrum, irradiance, and incremental dose series used during the MED assessment. The MED can be used as a quantitative measurement of the UVR dose required to cause erythema that is just perceptible to the naked eye, and in clinical practice, this is routinely used in many phototherapy units as a baseline measurement in each patient as an indicator of sensitivity to the phototherapeutic light source to ensure that the dose regimen used is individualized and safe. The MED is also used to identify abnormal photosensitivity and characterize the action spectrum for photosensitivity. Experimentally, the MED allows objective measurement of individual threshold erythemal sensitivity, thus enabling study conditions to be investigated, such as the effects of a photosensitizing drug, a photoprotective agent, of changing the characteristics of the light source, or of investigating parameters such as the influence of body site.
      Reflectance devices may also be employed to obtain quantitative data, defining the erythema index, in turn enabling the objective measurements of both threshold erythemal responses on the basis of MEDs but also the slope of dose-response curves and degree of induced erythema at doses above the threshold. It should be noted, however, that there are variations concerning visual or quantitative measurements of erythema, arising primarily from assessor training, ambient lighting conditions, and variability between measurement devices. In addition, owing to light absorption by melanin, reflectance devices are generally unreliable when used on darker skin phototypes or when there is significant UV-induced pigmentation. Typically, iterative provocation testing is performed at a larger test site than that used to obtain a MED, which is again important to document.
      Examples of phototesting options available for clinical studies are given as follows.
      Narrowband phototesting. This is only available through specialist photodiagnostic centers. Usually, an irradiation monochromator with a broadband xenon arc lamp is utilized. Light from this source is filtered by a diffraction-grating monochromator, allowing specific wavebands from the spectral range of the xenon arc lamp to be selected and used to irradiate the skin (
      • Moseley H.
      • Ferguson J.
      Which light source should be used for the investigation of clinical phototoxicity: monochromator or solar simulator?.
      ). Thus, the term monochromator is misleading because whereas the filtered emission spectrum is relatively narrow, it is not in fact a single-wavelength emission (typically full-width half maximum of 5–30 nm). These specific wavebands may be used to determine wavelength-specific MEDs. It is important to have a normal population reference range when measuring MEDs across the UV spectrum because it will allow deviations from the normal range of MEDs to be detected, although this normal range must be relevant to the population under study. Reporting must also include spectral emission and irradiances.
      Solar simulator. Despite the terminology, these light sources are often not a mimic of the solar spectrum (Figure 4). There is no agreed standard of what constitutes a solar simulator for clinical phototesting; therefore, it is important to understand the spectral output of the source being used and reporting using our core principles, especially if extrapolating findings to equivalent daylight exposures. The daylight spectrum is broadband and variable; therefore, care should always be taken in equating the results from solar simulator exposures to those from hypothetical daylight exposures. These devices may be used either to define MEDs or as a provocation source. Unfiltered solar simulators may miss UVA-weighted phenomena such as drug phototoxicity; therefore, care should be taken when interpreting wavelength-dependent mechanisms (
      • Moseley H.
      • Ferguson J.
      Which light source should be used for the investigation of clinical phototoxicity: monochromator or solar simulator?.
      ). Filters may be used to remove certain wavebands. Similarly, metal halide lamps are often used in combination with filters to achieve the desired spectral output. However, care should be taken, for example, erythema may be induced at ∼×10,000 lower dose with UVB than with long-wavelength UVA. Even residual exposure at certain wavelengths through the use of inefficient filters may confound results and create false positives (Figures 1 and 2).
      Figure thumbnail gr4
      Figure 4Comparison of daylight with a variety of solar simulators used for provocation testing in photobiology. AU, arbitrary unit.
      Often, erythemally weighted exposures from solar simulators are equated to a duration of exposure from daylight at solar noon in mid-June in terms of duration. In making such a comparison, however, the reference daylight spectrum used should be provided and understood in the context of the results presented.
      The skin’s response to UV irradiation varies depending on body site and skin phototype and is also influenced by chronic sun exposure, with a MED on a sun-exposed site typically being higher than that on a nonexposed area. For example, a MED on the lower leg is on the average four times higher than that on the trunk in lighter skin phototypes (
      • Olson R.L.
      • Sayre R.M.
      • Everett M.A.
      Effect of anatomic location and time on ultraviolet erythema.
      ). Provocation should ideally be carried out on consistent body sites (e.g., back) and on clear skin to allow for interpatient comparison. Alternatively, it is often most appropriately performed on a body site that may be of particular interest, for example, where a rash is induced with natural exposure. This information should always be included in reporting, and caution should be exercised if comparing results with a normal range, which may have been derived using a different irradiation source, body site, and/or patient population.

      Conclusions

      The effects of UVR on clinical and preclinical models are of fundamental importance. However, these underlying mechanisms may only be accurately interpreted through the practice of robust and reproducible UVR exposure. Increasingly, researchers are interested in the effects of visible and near-infrared radiation, and all of the principles outlined previously also apply when carrying out such exposures. Care should always be taken to ensure that irradiances and doses are biologically relevant and that appropriate experimental controls and safety precautions are observed.
      Reporting UV exposure is important in all preclinical and clinical experimental settings and in a transparent manner, which allows the reader sufficient information not only to understand fully the UV exposure carried out and its effects and to appropriately interpret the data but also to then be able to reproduce the exposures under the same experimental conditions.

      Conflict of Interest

      EE, VAM, RSD, and SHI provide unpaid consultancy to Spectratox, a not-for-profit company that offers commercial clinical and in vitro phototoxicity testing as well as UV meter calibration. RSD and SHI are also unpaid directors of Spectratox. PO states no conflict of interest.

      Multiple Choice Questions

      • 1.
        Of what benefit is a logarithmic plot of spectral irradiance?
        • A.
          It allows for calculations of uniformity of irradiance
        • B.
          It reveals smaller peaks in the spectrum, which may be more biologically effective
        • C.
          It allows the UV exposure to be better replicated
        • D.
          It can be used in the safety assessment of the lamp
      • 2.
        In an experiment, a sample is irradiated with a UVA lamp. The irradiance of the lamp is 4 W/m2, and the sample is irradiated for 1 minute. What is the total radiant exposure delivered in this example?
        • A.
          4 J/m2
        • B.
          240 J/m2
        • C.
          0.25 J/m2
        • D.
          15 J/m2
      • 3.
        Regulatory requirements state that any drug absorbing light between _________ nm must undergo photosafety evaluation.
        • A.
          100 nm and 400 nm
        • B.
          315 nm and 400 nm
        • C.
          290 nm and 700 nm
        • D.
          290 nm and 315 nm
      • 4.
        What is the main advantage of monochromators for phototesting?
        • A.
          They are portable
        • B.
          Any waveband from the spectral range of the xenon arc lamp may be selected for testing
        • C.
          The xenon arc lamp has a very long lifetime
        • D.
          They can irradiate multiple areas of the skin at once
      • 5.
        How is the minimal erythema dose defined?
        • A.
          The minimum dose of UVB radiation required to cause just perceptible reddening of the skin
        • B.
          The minimum irradiance of light required to cause just perceptible reddening of the skin
        • C.
          The minimum dose of light required to cause just perceptible reddening of the skin
        • D.
          The minimum dose of UVA radiation required to cause just perceptible reddening of the skin

      Author Contributions

      Conceptualization: SHI; Visualization: PO, EE; Writing - Original Draft Preparation: PO; Writing - Review and Editing: PO, VAM, RSD, EE, SHI

      Acknowledgments

      PO is funded by Medi-Lase (registered charity SC 037390) and the Alfred Stewart Trust.

      Detailed Answers

      • 1.
        Of what benefit is a logarithmic plot of spectral irradiance?
      • CORRECT ANSWER: B. It reveals smaller peaks in the spectrum, which may be more biologically effective
      • The logarithmic plot makes smaller peaks more visible, which may be more effective depending on the biological mechanism to be investigated. The normal spectral irradiance plot is sufficient for allowing the replication of experiments and for a safety assessment of the lamp. Neither plot is required for measuring the uniformity of irradiance.
      • 2.
        In an experiment, a sample is irradiated with a UVA lamp. The irradiance of the lamp is 4 W/m2, and the sample is irradiated for 1 minute. What is the total radiant exposure delivered in this example?
      • CORRECT ANSWER: B. 240 J/m2The radiant exposure delivered is equal to the irradiance (W/m2) multiplied by the exposure duration (seconds).
      • 3.
        Regulatory requirements state that any drug absorbing light between _________ nm must undergo photosafety evaluation.
      • CORRECT ANSWER: C. 290 nm and 700 nm.
      • Regulatory requirements state that any drug absorbing light between 290 nm and 700 nm must undergo photosafety evaluation. This range covers from UVB through the visible spectrum, encapsulating most of the primary wavebands of interest.
      • 4.
        What is the main advantage of monochromators for phototesting?
      • CORRECT ANSWER: B. Any waveband from the spectral range of the xenon arc lamp may be selected for testing
      • Monochromators allow the user to select specific wavebands from the xenon arc lamp for testing. They tend not to be portable devices, and the lamps are checked regularly and replaced when outputs decrease beyond a predetermined limit. They are typically only able to irradiate one area of skin at a time.
      • 5.
        How is the minimal erythema dose defined?
      • CORRECT ANSWER: C. The minimum dose of light required to cause just perceptible reddening of the skin
      • The definition of the minimal erythema dose (MED) is not specific to any waveband; however, a MED measured on the skin will be specific to the provocation source used. The MED is quantified as a dose of light (usually J/cm2), although irradiance would typically be recorded alongside a MED.

      Supplementary Material

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