By Michael Higlett, Marina Khazova, John O’Hagan
The continuing development of LED technology has resulted in the ever increasing use of LEDs in a wide range of consumer products, including toys. LEDs are used to a greater extent by toy manufacturers today to enhance toys visual appeal and choosing the appropriate LED is important to avoid the risk of harm to the child. 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. This led to the need for an easy to follow methodology for the assessment of the optical radiation safety of LEDs in toys [1] that may be adopted by toy manufactures when evaluating the suitability of an LED.The simplified approach is based on LED photometric characteristics from datasheets and avoids the need for additional complex measurements.
The LED selected for use in a toy should be ocular safe and the emission not greater than required for the intended purpose. The level of a child’s exposure to optical radiation during normal use and under foreseeable misuse should not exceed Exposure Limit values for the eyes and the skin recommended by the ICNIRP [2-4]. However, some ocular safe LED in a toy may cause temporary visual impairment.
Initial investigations of the temporary visual impairment (flash blindness, glare and distraction) from an ocular safe LED has begun using FAA [5] limits for lasers.
The assessment of visual impairment threshold is based on the accessible emission limit (AEL) [2-3] of the ocular safe LED. The AEL is a function of a peak wavelength, bandwidth and half width half maximum (HWHM) angle. Emission level of ocular safe LEDs at varying distances compared with flash blindness threshold is shown in Figure 1: it indicates the shortest distance where ocular safe LED doesn’t present the risk of flash blindness. It can be seen that the flash blindness limit varies from 16 cm to 87 cm depending on the HWHM angle; at longer distances there is no risk of flash blindness.
Figure 1: Comparison of emission of ocular safe LED (peak wavelength 500 nm, 40 nm bandwidth) with Flash Blindness threshold.
Table 1 shows the visual impairment distances for the LED presented in Figure 1, 60 degree HWHM angle only: there may be a risk of the flash blindness at the distances shorter than 87 cm, whereas then the glare may be an issue up to a further 3 m and distraction roughly 35 m further on.
Table 1: Visual Impairment threshold for LED with a peak wavelength of 500 nm, 40 nm bandwidth and HWHM angle of 60 degrees.
In conclusion, the ocular safe LED may cause temporary visual impairment including flash blindness at arm’s length. The visual impairment threshold depends on, peak emission wavelength and HWHM of the LED. It should be noted that the limits for temporary visual impairment are for dark conditions, while the majority of toys containing an LED would be used during the day as shown in figure 2. Assessment of the temporary visual impairment of the ocular safe LED for light conditions is required and is currently under investigation.
Figure 2: LED toy in light and dark conditions.
References
[1] Safety of Light Emitting Diodes in Toys. Journal of Radiological Protection, 32: 51-72; 2012
[2] 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.
[3] ICNIRP Guidelines on limits of exposure to broad-band incoherent optical radiation (0.38 to 3 mm). Health Physics, 73 (3): 539-554; 1997.
[4] ICNIRP Statement on light-emitting diodes (LEDs) and laser diodes: implication for hazard assessment.Health Physics, 78 (6), 744-752, 2000.
[5] Procedures for Handling Airspace Matters, JO 7400.2J. Federal Aviation Administration.