By: Michael Higlett, Marina Khazova, John O’Hagan

Health Protection Agency, UK

The development of technology has resulted in fast penetration of LEDs into a wide range of consumer products, including toys. The significant increase of optical output and expansion of the emission wavelength range, from ultraviolet to infrared, raised a concern about optical radiation safety of LED use in toys and led to the development of a methodology to assess the safety of the LED. The simplified approach is based on LED photometric characteristics from datasheets and avoids the need for additional complex measurements.

The emission of an LED in a toy should not be greater than required for the intended purpose. The level of a child’s exposure to optical radiation from an LED during normal use and under foreseeable misuse should not exceed Exposure Limit values for the eyes and the skin recommended by the ICNIRP [1-3]. The Accessible Emission Limit (AEL) values in this document establish a correlation between personal exposures under worst case exposure scenarios (distance, duration of the exposure) and the emission of the LEDs.  The AEL values are maximum level of LED emission not expected to result in overexposure to optical radiation under worst case scenarios. Two optical radiation hazards are considered to derive the AELs: UVA hazard to the eyes and Blue Light photochemical hazard to the retina of the eye, with the more restrictive AELs applied. Another consideration is that UV transmittance of the crystalline lens is much higher in infants under the age of 2 than in older children and the Blue Light spectral weighting for the infant eye is significantly higher up to 440 nm [1].

In the UV spectral range, the risk of adverse health effects to the eyes and the skin increases significantly at shorter wavelengths, especially below 315 nm. At the same time, the practical usefulness of short wavelength LEDs in toys to create visual effects is questionable. Therefore, on a risk/use balance, the use of LEDs emitting below 315 nm isn’t justified for children’s toys.

UVA-emitting LEDs are used in toys as excitation sources for luminescent materials, for example, “secret writing.” The UVA AEL includes only the LED emission within the spectral band of 315-400 nm. Because of the narrow-band emission and very low visual stimulus, the aversion response to the exposure from a UVA LED may be compromised and exposure in close proximity should be considered as foreseeable. A child’s curiosity triggered by fluorescence of the eye lens could increase the risk of such an exposure.

The AELs of LEDs emitting in the visible spectral range (300-700 nm) are defined by the more restrictive of the two exposure scenarios, UVA (as discussed above) or Blue Light photochemical hazard. For visible light,  AEL values are given in watts, which are spectrally weighted with the Blue Light hazard function, whereas in datasheets the LED output is often expressed in derivatives of un-weighted watts or in photometric units, such as candela or lumen [4].

Due to the different wavelength dependence of Blue Light and luminous efficiency weighting, the visible light AEL is wavelength and emission-bandwidth dependent if expressed in candela. The visible light AEL should also take into account the ICNIRP recommendation for a luminance limit of 10⁴ cd/m² and depends on the LED emission angle.

For LEDs with a peak emission wavelength below ~500 nm, the visible light AELs defined by the Blue Light hazard are more restrictive than the ICNIRP luminance requirements; above ~550 nm the luminance limit is more restrictive even for broad-band LEDs. The spectral bandwidth of the LED emission has a smaller effect on the visible light AELs than the peak emission wavelength.

More than 280 LEDs intended for incorporation into toys and a range of toys were measured. The majority of the tested LEDs could be considered as eye-safe for use in toys (see Fig.1). Measurements showed good agreement with the predictions of the simplified safety screening technique.

• Miniature red, orange and yellow LEDs could be considered eye-safe for use in toys, as a single component or in arrays;
• Low power green LEDs are safe for use in toys; high power green LEDs may require more detailed analysis, especially if used in arrays;
• The Blue Light hazard level of some of the blue LEDs is above ICNIRP ELVs, even for low power single components;
• Low power white LEDs could be considered as eye-safe; the Blue Light hazard level of the high power white LEDs may exceed the ICNIRP ELVs;
• Low power UVA LEDs are eye-safe for use in toys for children 3 years of age and older

 

References

[1] ICNIRP Guidelines on Limits of Exposure to Ultraviolet Radiation of Wavelengths between 180 nm and 400 nm (Incoherent Optical Radiation). Health Physics, 87 (2): 171-186; 2004.

[2] ICNIRP Guidelines on Limits of Exposure to Broad-band Incoherent Optical Radiation (0.38 to 3 mm). Health Physics, 73 (3): 539-554; 1997.

[3] ICNIRP Statement on light-emitting diodes (LEDs) and laser diodes: implication for hazard assessment. Health Physics, 78 (6), 744-752, 2000.

[4] CIE S 010/E: 2004 Photometry – The CIE system of physical photometry.