Blue light hazard – new recommendations on the assessment of LED lamps and luminaires

  1. Introduction

The EN 62471 standard is one of the most important documents discussing photobiological hazards. The standard defines principles which should be followed to assess photobiological hazards and safety of artificial and natural sources of optical radiation. It covers all kinds of sources emitting optical radiation in a very wide range of wavelength – in ultraviolet, visible spectrum and far infrared.

Presence near the emitting source, depending on its type, can lead to injuries and even to serious illnesses. Persons exposed to ultraviolet, visible or infrared radiation, can have the biological tissues of their eyes and skin damaged. Conjunctivitis, keratitis and erythema are frequent consequences of ultraviolet radiation. Long term UV radiation can damage eyes and skin and lead to such problems as cataract, skin ageing and skin cancer.

Equally affected are persons exposed to intensive visible or infrared radiation. Visible radiation, and particularly its thermal and photochemical mechanisms, can damage eye retina. Even short term infrared radiation, with high radiation intensity, can damage the external layer of eyes (cornea) and lead to premature ageing of the skin.

For these reasons the measurement range imposed by the standard is wide, from 200 to 3000 nm. Therefore, assessment of photobiological hazard becomes a complex metrological problem, which requires specialist calibrated measurement equipment and extensive technical skills of laboratory personnel. The measurement equipment used to assess hazard should have high resolution and must be properly calibrated.

 

  1. Assessment of the hazard caused by LED lamps and luminaires

LED lamps and luminaires used for general lighting purposes and in industrial applications emit mainly optical radiation in the visible range. Therefore, unlike lamps of other types, LED lamps and luminaires create mainly photobiological blue light hazard. Assessment of photobiological safety of blue light emitting sources and luminaires presented in EN 62471:2010 [1] and extensively described by Pietrzykowski [2] covers lamps and luminaires:

 

  • with continuous or pulse light, large and small, as per the classification given in the standard,
  • intended for general lighting purposes and industrial applications,
  • with uniform or non-uniform spatial distribution of radiation intensity.

In order to assess  blue  light  hazard, either  the  effective  value of blue  light  irradiance EB, or the effective value of blue light radiance LB is determined, depending of the source size, see Table 1.

 

Table 1.  Blue light hazard to eye retina

Hazard Measured value Geometry Spectral range

[nm]

Blue light hazard to the eye – a small source with angular size a < 0,011 rad Blue light irradiance EB [W×m-2] EB – Irradiance method 300 – 700
Blue light hazard to the eye – a source with angular size a ³ 0,011 rad Blue light radiance LB

[W×m-2×sr-1]

LB – Radiance method 300 – 700

 

Blue light irradiance EB [W×m-2] in the range 300 to 700 nm can be measured by a high class spectroradiometer equipped with measurement head with properly selected angular correction. Fig. 1 shows GL SPECTIS 5.0 UV VIS spectroradiometer with an operating range of 200 to 800 nm. The spectroradiometer is calibrated with spectroradiometer standards: deuter discharge lamp for UV and tungsten-halogen lamp for UVA, visible and near infrared areas. Such measurement equipment is used at the The Centre for Research and Development on Work Processes and Safety Engineering in Warsaw, Poland to assess photobiological hazard.

Fig. 1. High class GL SPECTIS 5.0 UV VIS 200 to 800 nm spectroradiometer with the spectral irradiance measurement head

A dedicated measurement probe, namely a telescope connected with the spectroradiometer by fiber optic, can be used to assess blue light radiance LB [W×m-2×sr-1] in the range 300 to 700 nm [Fig. 2].

Fig. 2. Radiance Telescope – an attachment to measure radiance, connected to the spectroradiometer with fiber optic

 

  1.  Technical Report IEC TR 62778:2014
3.1          General

Standard PN-EN 62471 was supplemented with Technical Report IEC TR 62778:2014 [3], which contains extremely important explanations and guidelines on the assessment of blue light hazard emitted by lighting products, which emit mainly optical radiation in the visible area. Using optical and spectral calculations, the report explains that measurements of photobiological safety described in IEC 62471 inform us about a product, if the product is intended to be used as a component of a more complex lighting product, and how this information can be transferred from the component of a product to a complex product.

In many countries, including in Poland, IEC technical reports, are made part of the generally applicable standards, and for this reason there are no normative documents. Fortunately, this situation will be improved shortly since the International Electrotechnical Commission decided to replace the Technical report with an international standard. The first draft of the standard has already been published [4].

3.2          New terms and definitions

As stated above, many light sources used for general lighting purposes emit mainly visible radiation. All these sources, to a larger or lesser extent, create blue light photobiological hazard.

As spectral ranges of visible radiation and the function of blue light hazard efficacy overlap, we can find relations connecting photometric and colorimetric quantities with quantities of blue light hazard efficacy B(λ), and in this way we get additional information about lamps or luminaires (particularly about white light LED lamps and luminaires).

The Report introduces a few new physical terms and their definitions. The terms are used to assess blue light hazard and they include:

  • blue light hazard efficacy of luminous radiation KB,v

Definition:  quotient of blue light hazard quantity (e.g. EB or LB) to the corresponding photometric quantity
[W×lm-1]

or an equivalent relation:

[W×lm-1]
where:

E
λ(λ) –  spectral irradiance,

Lλ(λ) –   spectral radiance,

B(λ)  –   relative blue light hazard efficacy,

V(λ)  –   spectral luminous efficiency for photopic vision,

Km       –   683 lm×W-1

EB        –  blue light irradiance,

Ev        –   illuminance,

LB        –   blue light radiance,

Lv        –   luminance.

 

  • blue light hazard efficacy of radiation

Definition:  quotient of blue light hazard quantity (e.g. EB or LB) to the corresponding radiation quantity:


or an equivalent relation:


where:
Ee – irradiance,

Le – radiance.

 

  • threshold illuminance Ethr

Definition:   threshold illuminance value, below which the light source can never give rise to an exposure time tmax < 100 s, regardless of the light source’s LB value

 

  • threshold distance dthr

 

Definition:     distance from the light source at which the illuminance produced by that light source is equal to the Ethr value for that light source

 

  1. Assessment and measurement of blue light hazard

 

4.1          An overview of methods

Assessment of blue light photobiological hazard usually requires time consuming measurements. However, correlations existing between photometric and colorimetric quantities and blue light hazard effective quantities help to simplify the assessment in certain cases. As a result of the assessment light sources are grouped into RG0 or RG1 hazard groups; if light sources are classified as RG2 hazard group, then threshold illuminance Ethr is determined.

 

Presently the following assessment methods of blue light hazard are used:

 

  1. assessment based on measurements of photometric and colorimetric quantities,
  2. assessment based on measurements of spectral quantities and luminance
  3. assessment based only on measurements of spectral quantities and relevant calculations

 

4.2          Assessment based on measurements of photometric and colorimetric quantities 

The method uses relations existing between quantities which describe blue light hazard and photometric and colorimetric quantities of a light source and is applied to white light sources only. When correlated color temperature Tcp of the light source is calculated, the corresponding value of threshold illuminance Ethr is found in Table 2.

 

Table 2. Conservative assessment of Ethr as the function of correlated color temperature Tcp

 

Nominal Tcp

[K]

Conservative values of Ethr

[lx]

CCT £ 2 350 K 4 000
2 350 K < CCT £ 2 850 K 1 850
2 850 K < CCT £ 3 250 K 1 450
3 250 K < CCT £ 3 750 K 1 100
3 750 K < CCT £ 4 500 K 850
4 500 K < CCT £ 5 750 K 650
5 750 K < CCT £ 8 000 K 500

 

 

If correlated color temperature Tcp and source luminance Ls are known, we can also determine the conservative value of luminance L using Table 3, and compare it with Ls. If Ls < L then the light source belongs to hazard group RG1 unlimited, whereas if Ls ³ L, Ethr must be determined as before.

 

Table 3. Conservative assessment of luminance values producing hazard group lower than RG1

 

Nominal correlated colour temperature [K] Conservative values of luminance L  [Mcd/m2]
CCT ≤ 2 350 K 40
2 350 K < CCT ≤ 2 850 K 18,5
2 850 K < CCT ≤ 3 250 K 14,5
3 250 K < CCT ≤ 3 750 K 11
3 750 K < CCT ≤ 4 500 K 8,5
4 500 K < CCT ≤ 5 750 K 6,5
5 750 K < CCT ≤ 8 000 K 5
Nominal values of correlated color temperature and luminance given by the manufacturer can be used as the basis for the assessment.

 

 

4.3         Assessment based on measurements of spectral quantities and luminance

The method can be used if measurements of spectral quantities and luminance of the source or luminaire Ls are available. In the case of non-uniform luminance distribution of luminance must be measured and the highest value of luminance at 200 mm and field of vision 0.011 rad should be determined, followed by measurement of the relative distribution of spectral irradiance Eλ(λ). Using the formula for blue light hazard efficacy for visible radiation, we calculate KB,v and blue light radiance using the formula

L=Ls×KB,v                                                                                                                                                                                                                               

If LB < 10 000 W/(m2×sr), then the source belongs to hazard group RG1 unlimited, whereas if LB > 10 000 W/(m2×sr), Ethr should be calculated using the formula

 

Ethr = 1 W×m-2 / KB,v

 

Determination of threshold illuminance Ethr and threshold distance dthr is presented in Section 5.

 

4.4     Assessment based only on measurements of spectral quantities and relevant calculations

The method is very similar to the method discussed in 4.3 and can be easily used with spectroradiometer. After spectral radiance at the distance of 200 mm and field of vision of 0.011 rad have been measured, values of LB and KB,v and Ethr are calculated.

 

  1. Determination of threshold illuminance Ethr and threshold distance dthr

 Assessment of blue light hazard for sources with maximum angular size 11 mrad using a spectroradiometer begins with measurements of spectral radiance at measurement distance of 200 mm and field of vision 11 mrad.

If the spectroradiometer used to make the measurements is calibrated in values of spectral irradiance, the values must be converted using the following formula:

 


where:   Le(λ) –  spectral radiance, W×m-2×sr-1×nm-1,

Ee(λ) –   spectral irradiance, W×m-2×nm-1,

Ω        –   solid angle, sr,

r           –   measurement distance, m,

A          –   aperture area, m2.

 

On the basis of values Lel we calculate the value of blue light radiance LB using the formula:

 

[W×m-2×sr-1]

 

where:    B(λ) – function of spectral blue light efficacy for eye retina

 

If LB is smaller than 10 000 W×m-2×sr-1 the hazard group of the source is not greater than RG1, whereas if LB is greater than 10 000 W×m-2×sr-1, the source belongs to hazard group RG2 and it is necessary to calculate threshold illuminance Ethr. First source luminance must be calculated, using the formula:

 

[cd×m2]

 

It follows from the definition of threshold illuminance Ethr that for the value E < Ethr a light source will be moved to hazard group RG1.

 

The formula used to calculate threshold illuminance is:

 

Ethr = 1 W×m-2 / KB,v = 1 W×m-2×Lv / LB

 

Example:

For a certain LED source the following values were obtained:  LB = 19624,8  W×m-2×sr-1; Lv = 15465113 cd×m-2. Substituting these values to formula we get

 

The values of threshold distance dthr can be obtained by one of the following methods:

 

  • using a illuminance meter with a correctly inclined receiver, when the direction of maximum light intensity can be easily determined,

 

 

Fig. 3. Measurement setup to determine threshold distance dthr of sources with directional (point) distribution

 

 

  • using a goniophotometer, when the direction of maximum light intensity is not known

 

In the first case, for point sources, the measurement setup has a very simple configuration, shown in Fig. 3, whereas in the latter case, when a goniophotometer is used, first light distribution is  determined and then the direction of maximum light intensity Imax and threshold distance dthr are calculated from the formula

 

 

  1. Blue light hazard in standards on light sources and luminaires

Changes in the assessment of blue light hazard introduced by Technical Report TR 62778 also apply to standards on light sources and luminaires. IEC 60598-1:2014 [5] is a notable example here – it contains general requirements and assessments of luminaires but also refers to problems connected with blue light hazard (cf. section 4.24.2).

Luminaires with light sources which are not excluded from the assessment of blue light hazard for eye retina should be assessed in accordance with IEC/TR 62778.

The use of light sources in hazard groups higher than RG2 (with respect to blue light) is not expected. This would require satisfaction of tough requirements on the use of this type of light sources. Presently, the types of light sources which should be considered with respect to blue light hazard include only LED lamps, metal-halogen lamps and some types of special halogen lamps.

No requirements on blue light hazard for eye retina are applied to luminaires which use a light source in hazard group RG0 unlimited or RG1 unlimited, in accordance with IEC/TR 62778, or which were assessed as RG0 unlimited or RG1 unlimited after they have been completely installed, under the same conditions.

 

The following requirements apply to luminaires with threshold illuminance Ethr determined in accordance with IEC/TR 62778:

 

  1. Permanently installed luminaires – perform an additional assessment in accordance with IEC/TR 62778 in order to find distance x between the luminaire and the border line between RG2 and RG1. The luminaire should be marked and carry an instruction with standard requirements.
  2. Portable and manual luminaires with hazard groups higher than RG1 at 200 mm are assessed in accordance with IEC/TR 62778 and they should be marked as stipulated in the standard.

 

More information on instrumentation by GL Optic for blue light hazard assessment can be found here

 

Literature

 

[1]      EN 62471:2010   Photobiological Safety of Lamps and Lamp Systems

[2]       Pietrzykowski J.: Bezpieczeństwo fotobiologiczne sztucznych i naturalnych źródeł promieniowania optycznego. Część 1: LUX Magazyn nr 2, 54-57 (2015). Część 2: LUX Magazyn nr 3, 59-66 (2015).

[3]       IEC TR 62778:2014   Application of IEC 62471 for the assessment of blue hazard to light sources and luminaires.

[4]       34A/1887/DC Draft for an International Standard IEC 62778 to replace the Technical Raport IEC/TR 62778 (2016).

[5]       IEC 50598-1:2014   Luminaires. Part 1: General requirements and tests.

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