7.1 Phototherapy and Radiation Protection

See here a shorter overiew of light hazard applications and corresponding products:

See here a short overview of photo medicine with corresponding products:

Incoherent Optical Radiation Protection

Even though there are many wide ranging and highly positive effects of light, there are also negative effects to consider. Naturally occurring optical radiation, especially in the UV range of the solar spectrum, poses a potential health risk to outdoor workers and people who spend a significant amount of time outdoors. The most serious long-term consequence of UV exposure is the formation of malignant melanoma of the skin, a dangerous type of cancer. In the US, skin cancer is the most frequently contracted type of cancer, and since the 1970s, the incidence rates of malignant melanoma have more than doubled. The same applies for other countries and as a result, the national and supranational networks of solar UV detectors have recently been established to monitor solar UV levels. In addition, the World Meteorological Organization is currently preparing guidelines for their characterization, calibration and maintenance.

Fig. 1: Incidence rates of malignant melanoma in the US since 1973

In simple terms, incoherent optical radiation is optical radiation in the range of wavelengths between 100 nm and 1 mm, other than that emitted by lasers. The effect of incoherent optical radiation on the skin and the eye is increasingly being studied. The reasons for this lie in the rising exposure to radiation from sunlight, particularly in the UV range, and the growing use of high powered lamps in radiation therapy, radiation cosmetics, UV radiation curing, UV sterilization, vehicle headlamps, lighting equipment, etc. The high proportions of UV and blue light in the emission spectra of these lamps can, in addition to their desired effects, also result in radiation damage through both direct and indirect contact if the maximum permitted exposure levels are exceeded.

The shallow depth of penetration of optical radiation restricts the health hazards primarily to the eye and skin.

Fig. 2: Optical radiation health risks

Relevant Radiation Quantities

When evaluating the harm that might be caused by incoherent optical radiation, it is the effective radiance (or the time integral of the radiance) that is critical for the retina. On the other hand, the effective irradiance (or the exposure, also known as dose) is the critical quantity for the skin, cornea and eye lens. The exposure can for instance arise be at a workplace.

Photobiologically effective radiance

W/(m²sr)

Lbiol =	∞	∫ Leλ(λ) × s(λ)biol, rel × dλ
	0

with L_eλ(λ): spectral radiance of the radiation sources

Photobiologically effective irradiance

W/m²

Ebiol =	∞	∫ Eeλ(λ) × s(λ)biol, rel  × dλ
	0

 E_eλ(λ) is the spectral irradiance of the radiation sources

Photobiologically effective exposure

dose, J/m²

Hbiol =	t1	∫ Ebiol × dt
	0

s(λ)_biol, rel stands for the relevant spectral response functions of the skin and eye.

The following conditions should be maintained if exposure limits (E_limit) are given in guidelines as effective radiance, limit, or effective irradiance:

E_biol ≤ E_limit or L_biol ≤ L_limit

If the exposure values are given as the time integral of the radiance L_i or as the exposure (dose), H, then the maximum permissible exposure duration, t, can be calculated:

t = L_i / L_biol oder t = H / E_biol

ACGIH / ICNIRP Spectral Weighting Functions for Assessing UV Radiation Hazards

The spectral weighting function for the acutely harmful effects of UV radiation was developed by the American Conference of Governmental Industrial Hygienists (ACGIH) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP).

If one examines the spectral curve describing this function, it is seen that the spectral effectiveness in the UV-C and UV-B ranges is very high, and that it falls drastically in the UV-A range. The reason for this is that the function is derived from the functions relating the radiation to erythema (skin reddening) and photo-kerato-conjunctivitis (corneal inflammation). The range of wavelengths from 315 nm to 400 nm (UV-A) corresponds to a rectangular function representing total UV-A. Threshold limit values given for the maximum permissible exposure of the skin define the range of wavelengths as 200 nm (180 nm) to 400 nm in reference to the ACGIH-ICNIRP function. The limits of maximum permissible exposure for the eye in the 200 nm (180 nm)to 400 nm range and 315 nm to 400 nm (UV-A) are defined separately. According to the ACGIH-ICNIRP definition, UV-C/B is measured in effective irradiance according to the spectrally weighted function and the UV-A level is assessed by measurement of the total UV-A irradiance (no spectral weighting function) for UV-A rich sources.

Fig. 3: ACGIH spectral function

Blue-Light Hazard and Retinal Thermal Hazard Functions for Photochemical and Thermal Risks to the Retina

Blue Light Hazard (BLH)

If optical radiation with wavelengths between 380 nm and 1400 nm of sufficient intensity reaches the retina, it can cause photochemical and thermal injury. Radiation in the “blue” part of the spectrum between 380 nm and 700 nm (effectively 380 nm to 550 nm) triggers photochemical reactions. If the photon energy in the radiation is high enough, it converts chemically unstable molecules into one or more types of molecules. The spectral curve of the blue light hazard response function is shown in the following diagram. ICNIRP 1997 gives the following limits for the effective radiance of the BLH function:

L_BLH × t ≤ 100 J × cm^-2 × sr^-1 for t ≤ 10.000 s

L_BLH ≤ 10 mW × cm^-2 × sr^-1 for t > 10.000 s

L_BLH = effective radiance
t = duration of exposure

The blue light hazard function generally applies to exposure periods of more than 10 s. For shorter exposure times, the thermal retinal injury function applies.

Fig. 4: Blue light hazard spectral function

Thermal Injury to the Eye (RTH Retina Thermal Hazard)

If the retina is exposed to high radiation intensities for short periods, a temperature rise to 45 °C leads to hyperthermia, a 60 °C rise causes coagulation, and a rise of over 100 °C results in vaporization. Cooling down the heat mostly depends on the capacity of the irradiated zone to transfer heat, and thus on the size of the image of the radiation source on the retina. The diagram above illustrates the spectral response function for thermal damage to the retina according to ICNIRP.

In the spectral range between 380 nm and 500 nm, the effect of the RTH function is larger than the BLH function by a factor 10. Whereas the latter rapidly falls to zero for wavelengths above 500 nm, the thermal function continues on for wavelengths up to 1400 nm. Since there are no industrially useable radiation sensors with spectral sensitivity for wavelengths ranging from 380 nm to 1400 nm, an appropriately simulated match using silicon photodiodes is used. In this context, it is adequate to measure the range up to 1200 nm since various light sources exhibit no more than 4 % difference in the integrated totals for wavelengths up to 1200 nm and to 1400 nm. This statement is also confirmed by ICNIRP in their working paper /1/.

For radiation sources whose emissions lie primarily in the near infrared range (IR-A) between 780 nm and 1400 nm, and that generate a visual luminance of less than 10 cd/m², the visual stimulus is so weak that the aversion reflex is not activated. In such applications, the measurement of radiance must, according to ICNIRP, take place exclusively in the IR-A region.

L(λ): spectral radiance of the radiation source being measured
RTH(λ): retina thermal hazard function
α: apparent radiation source.

Limits are also prescribed for the RTH function. Thus, for the case where

10 μs ≤ t ≤ 10 s

L_haz ≤ 50 / ( α × t^0.25 ) ( kW × m^-2 × sr^-1 )

L_haz = effective radiance for the RTH function
α = size of the light source expressed in radians

For t < 10 μs, the limit must not be larger than L_haz for t = 10 μs. For t > 10 μs the limit must not be larger than L_haz for t = 10 s.

Metrological Considerations

Radiance is the quantity relevant to the evaluation of BLH and RTH hazards. The latest draft standards (IEC 825-1, November 1998) and ICNIRP (printed in Health Physics 1999) express views as to the angle of the measurement field of radiance meters. The applicable figures related to exposure durations are:

t < 10 s and α = 1.7 mrad ^*

t = 10 s … 100 s and α = 11 mrad ^**

t = 100 s … 10000 s and α = 1.1 × t^0.5 mrad / s^0.5 **

t > 10000 s and α = 100 mrad ^**

^* Dominance of thermal damage to the retina
^** Dominance of blue light hazard

 Fig. 5: Light Source – Subtended Angle – Retina

For RTH IR-A, evaluation ANSI/IESNA RP-27.1-96 recommends a field of view of 11 mrad, and of 100 mrad for very large radiation sources. 

/1/ ICNIRP: guidelines of limits of exposure to broad-band incoherent optical radiation (0.38 μm to 3 μm) (September 1997).

Fig. 6: Retinal thermal and blue light hazard spectral functions

UV-Erythema

The typical symptom of UV erythema is acute skin inflammation caused by UV radiation (sunburn). It was previously thought that erythema was only caused by radiation components in the UV-B range of wavelengths. Present opinion is that UV-A plays a part in causing erythema because there is so much more of it present. Medical investigations have shown that intensive exposure to UV in leisure time and at work increases the risk of skin cancer. Children in particular should be protected from strong UV radiation since the skin stores the information about the UV dose received in the first years of life, and this can be a leading factor in the development of skin tumors in later years.

Sunburns occur in fair-skinned people (skin type 2) with a UV dose of as little as 250 J/m². Our table (according to F. Greiter: Sonne und Gesundheit, (Sun and Health), published by Gustav Fischer Verlag 1984) lists the various exposure duration's for minimal skin reddening for different skin types.

Fig. 7: Erythemal spectral function

Tab. 1: Skin type categories

Phototherapy

UV-A, UV-B and UV-B311 Phototherapy

UV is widely used by dermatologists in the treatment of certain skin diseases like Psoriasis and Vitiligo. Whole body exposure booths as well as hand and foot units that use light sources which emit broadband UV-A, UV-B, narrowband 311 nm UVB and combinations of UV-A and UV-B are used to irradiate the patient.

In PUVA phototherapy, also called photochemotherapy, UV-A is applied in combination with a photosensitizing agent. The photosensitizing agent is taken in pill form or applied topically to the skin. This medication is called psoralen, hence the acronym PUVA, and makes the skin more sensitive and responsive to the UV-A (315 nm – 400 nm) wavelengths.

Due to the risks of premature skin ageing and skin cancer from prolonged exposures, also with consideration to skin type, PUVA is only recommended for moderate to severe cases of Psoriasis. As a side note, psoralen is currently also used as a photosensitizer in UV sterilization of blood.

UV-B broadband treatment is normally administered without a photosensitizing agent. It is considered safer than UV-A for wavelengths between approx. 290 nm to 315 nm since it does not penetrate as deeply into the skin and is more energetic thus allowing for shorter exposure times. However, it is generally accepted that wavelengths below 290 nm produce more erythema, which can actually inhibit the therapeutic effects of the longer wavelengths.

As a result, narrowband UV-B sources emitting predominantly at 311 nm – 312 nm have been developed. They emit right in the wavelength zone that is most effective while producing lesser erythemal interference than broadband UV-B sources.

This is generally known as a TL-01 source. A TL-12 UV-B source with a slightly wider band of emittance between 280 nm – 350 nm, peaking at about 305 nm, is also used. For more information, contact the National Psoriasis Foundation and the American and European Academies of Dermatology.

Fig. 8: Narrowband 311 nm and broadband UV-B source spectra

Dose, used here as irradiance accumulated over time, is normally measured in phototherapy applications.

joules / cm² = watts / cm² × seconds

dose / energy = irradiance × time

In the research and development stage or field service, direct irradiance may be monitored to discern any variation in output through lamp or delivery system degradation. Most of today’s phototherapy instruments are howevery equipped with sensors and electronics that enable delivery of pre-selected UV doses.

Third party checks of these internal dosimeters using qualified UV radiometers is recommended to ensure proper dosimetry and safety.

Bilirubin Phototherapy

Newborn jaundice or neonatal hyperbilirubinemia, a yellowish appearance of the skin and whites of the eyes, is present to some degree in almost all newborn infants. This is caused by an elevated level of bilirubin molecules in the blood, which is as a result of immature liver function combined with the destruction of the present red blood cells present. When these levels are very high, one method of clearing the jaundice is by exposing the newborn to light in the blue spectral region between 400 nm and 550 nm. The light interacts with the bilirubin, converts it to a substance that is excreted back into the bloodstream before being excreted with feces. The naked newborn is placed in a ‘bilibed’ or protected Isolette and exposed to fluorescent lights designed or filtered to emit in the blue spectrum. A recent development is the ‘biliblanket’ that delivers blue light through fiber optics and can be wrapped around the infant. Radiometric measurements of bili-lights are important in order to ensure proper dosimetry.

Efforts to standardize an action spectral function and measurement procedures for bilirubin are in process. Due to early work in this field, the units of microwatts/cm²/nm were wrongly adopted for radiometric measurement of bili-lights. For technical correctness, the units of watts/cm² should be used.

Fig. 9: Bilirubin spectral function

An different application are weatherproofed measurement to determine the UV-Index (erythemal weighted dimensionless quantity):