Photobiological safety measurements
The hazardous effect of laser radiation to the human eye and skin is studied for a long time and several standards and by-laws regulate the handling of lasers. Today more and more attention is payed to the effect of incoherent optical radiation on the human body. The reason is the rapid progress in development and manufacturing of new bright light sources, mainly based on LEDs.
The main interactions with regard to hazard impact between optical radiation and the human being are the following:
- Photobiological effects of the UV part of spectrum to eyes and skin (Actinic UV and UVA)
- Hazardous effect, especially of the blue part of light spectrum to the human eye (BLH)
- Thermal effect of radiation to human skin and the retina (skin and retina thermal)
- Effect of IR to the human eye (Eye IR)
The European Union issued the directive 2006/ 25/ EC, which defines the obligations for employers to protect the employees against health injuries caused by incoherent optical radiation.
This directive was transferred into national laws.
Additional main documents related to the topic are:
- AOR Guide 2006/25/EC (Explanation of the directive)
- ANSI/ IESNA RP-27.1 and RP-27.2 (Recommended practice)
- IEC 62471 (Photobiological safety of lamps and lamp systems)
- IEC 62778 (Application of IEC 62471 for the assessment of blue light hazard to light sources and luminaires)
- CIE S007/ ISO 17166 (Erythema Reference Action Spectrum and Standard Erythema Dose)
IEC 62471 is the basic standard for the measurement of photobiological hazard caused by optical radiation. It is currently revised by a JTC of CIE and IEC. Additionally many national and international projects deal with the improvement of standards as well as the simplification and standardization of the measurement procedures. At present, there is still a great deal of confusion about the implementation of measurements for the classification of radiation sources.
The sources are divided into four risk groups:
- Excempt group: The source can never cause photobiological damage.
- Risk Group 1 (low risk): The source does not pose a hazard due to normal behavioral limitations on exposure.
- Risk Group 2 (Moderate Risk): This source does not pose a hazard due to the aversion response to very bright light sources or due to thermal discomfort.
- Risk Group 3 (High Risk): The source may pose a hazard even for a brief exposure.
The standards define Exposure Limits (EL) and related conditions. The classification can be done by calculations using radiometric data of the sources. If this is not possible to a sufficient degree or if data are missing, specific measurements must be carried out.
The CIE published a position paper in 2019 stating that light sources of daily use, even if they are equipped with LEDs, cannot lead to photobiological hazards.
Typical light sources for which an assessment of their hazard potential must be made are studio spotlights, UV lighting in technological systems and lighting for image processing.
The following tables shows the different hazard categories of IEC 62471/ ANSI/ IESNA RP-27 and their measuring conditions more in detail. The values related to the surface of the skin and to the cornea are Irradiance based and the values related to the retina are Radiance based. The radiance measurements can be in contradiction to basic rules in classical light measurement. This is the case, if the source is smaller than the viewing angle of the meter. Some of the values are related to the pure radiometric measurements, others have to be weighted with special efficiency functions.
Irradiance based measuring values
|Hazard name||Symbol||Wavelength range /nm||Weighting function||Remark|
|Actinic UV (Skin and ocular exposure)||ES||200 - 400||S(λ)|
|Eye UVA (Ocular exposure)||EUVA||315 (320) – 400||-||Different limits below and above 1000 s|
|Eye IR (Ocular exposure)||EIR||780 - 3000||-||Different limits below and above 1000 s|
|Skin thermal exposure||EH||380 - 3000||-||Limited to small skin areas|
|Retinal blue light exposure – small source||EB||300 - 700||B(λ)||For sources with angular appearance smaller 11 mrad, in ANSI/ IESNA also aphatic (A(λ)), different limits below and above 100 s|
Radiance based measuring values
|Hazard name||Symbol||Wavelength range /nm||Weighting
Blue light hazard (BLH)
|LB||300 - 700||B(λ)||
in IESNA also aphatic (A(λ)), different limits below and above 1000 s
Retinal thermal exposure
|LR||380 – 1400||R(λ)||
Subtended angle of source needed
IR radiation exposure – weak visual stimulus
|LIR||780 - 1400||R(λ)||
Subtended angle of source needed
The radiometric calculation or measuring results must be transferred into exposure values using the real exposure times. Afterwards the results must be compared with the limits of exposure for the different effects, defined in the standards. Vice versa is it possible to calculate the maximum exposure time for an employee in this radiation situation.
The measurement can be the more reliable choice, but it is necessary to use a suited measuring instrument. It can be carried out by filter instruments (detector with appropriate filter to obtain the defined sensitivity characteristics) or a spectroradiometer, where the weighting is done by calculation. The latter solution is the more flexible one, because the weighting functions can easily be changed by the software. The mentioned documents recommend the usage of a double monochromator system, but such instrument is not easily portable.
A more simple spectroradiometer can be used, if one has a good knowledge of the application and when the parameters of the instrument are carefully considered, especially the stray light, the wavelength accuracy, the viewing angle, the bandwidths, the specification of the input optics (homogeneity, polarization influence) and the noise limit (specified by the noise equivalent input).
The following figures show the measurement of a welding arc with a specbos 1211UV-2 as well as the result (radiometric as well as weighted by the S(λ) and B(λ) curves).
Measurement of a welding arc with specbos 1211UV-2
IESNA RP-27.2-00, IEC 62471 and ISO 17166 describe different measurement geometries for the different hazard effects.
The Radiance based measurements must be done with a direct viewing optics (small viewing angle, radiance mode), the irradiance ones with an optics viewing in the half space (viewing angle 180° = 2π, measurement with diffusor or integrating sphere, irradiance mode). The exposure result will be obtained by integration of the weighted spectrum and multiplication by the exposure time (presumed that the intensity of the radiation is constant during the exposure).
The figure shows the three weighting functions S(λ), B(λ) and R(λ), additionally the aphatic (lenseless) A(λ) function, which is only included in the US document ANSI/ IESNA RP-27.2-00.
The meaning of the functions is as follows (see table above):
UV Hazard Weighting Function
|Skin and eye injury caused by UV radiation
Blue-light UV Hazard Weighting Function
|Photochemically induced retinal injury induced by blue radiation
Burn Hazard Weighting Function
|Thermal damage of the eye, especially of retina and lens
Aphakic Hazard Weighting Function
Mainly retinal injury of the lensless eye
Photobiological safety measurements by using the JETI LiVal software
The software JETI LiVal includes the Hazard measurements in a special calculation window. It shows the limits of the different groups for the selected category, calculates the final measuring value and shows the classification into the risk groups.