Infrared LEDs are used in eye trackers, security cameras, depth sensors, and many other devices, and properly designed systems can use them safely and effectively. Because infrared light is invisible, it is helpful to verify the output of unfamiliar or modified emitters. This guide explains the factors that affect exposure and how to evaluate an infrared setup.
Infrared lighting is a practical and widely used part of many commercial eye trackers. Most low-power systems are designed to keep exposure within appropriate limits. Still, infrared sources can vary, and strong exposure can heat tissue in the eye. Because near-infrared light is invisible, measuring an unfamiliar setup is more reliable than judging its brightness by sight or camera phone.
Irradiance describes radiant power arriving at a surface per unit
area. It is commonly measured in watts per square meter
(W/m²) or milliwatts per square centimeter
(mW/cm²).
10 W/m² (1 mW/cm²)
IEC 62471
provides a framework for evaluating the photobiological
safety of lamps and lamp systems, including infrared sources. A value
such as 100 W/m² (10 mW/cm²) can be useful as
a simplified screening reference in a demonstration, but it is not a
universal pass-or-fail limit for every infrared LED.
A proper assessment depends on the emitter wavelength, exposure duration, viewing distance, beam shape, apparent source size, and the relevant hazard being evaluated. Consumer meters may also have limited wavelength ranges or may respond differently to sunlight and narrow-band LEDs. For any DIY system or product, assess the complete system against an objective standard in repeatable conditions.
An infrared irradiance meter
can help compare sources under a consistent test setup. In one
informal experiment, a dark room produced a near-zero reading, a
monitor measured about 1 W/m² (0.1 mW/cm²),
and a fireplace measured approximately
10-15 W/m² (1-1.5 mW/cm²).
Indoor daylight varies substantially with the room and the direction
of light. Irradiance readings near windows range from several watts per
square meter to more than
100 W/m² (10 mW/cm²). Outdoor daylight
produces much higher meter readings in the 500+ W/m² (50 mW/cm²)
range, especially in direct sunlight.
These examples show why distance, direction, environment, and meter
range should be recorded with every measurement.
Do not look directly at the sun!
The optical shape of an LED affects how its output is distributed. A domed LED can concentrate light into a narrower beam, while a flat-top or diffused LED spreads the output across a wider area. A coating can diffuse it further.
At a distance of five centimeters, three LEDs powered by the same OV9281 camera module produced the following example readings:
Domed LED: approximately 10-15 W/m² (1-1.5 mW/cm²)
Sanded flat-top LED: approximately 3-4 W/m² (0.3-0.4 mW/cm²)
Painted flat-top LED: approximately 1-2 W/m² (0.1-0.2 mW/cm²)These readings illustrate the effect of diffusion, but they do not establish one limit for every distance or exposure duration. Diffusion and sensible placement can help reduce concentrated output, while moving an emitter closer to the eye increases irradiance. If you modify an LED, measure the finished setup so you know how it performs.
LED output is strongly related to driving current. Two products using visually identical LEDs can produce very different irradiance if their circuits supply different current or use different pulse patterns.
Checking average current, peak pulse current, and duty cycle makes it easier to choose an appropriate emitter and compare setups. Reliable current limiting also helps keep output predictable during use.
Use an irradiance meter such as the one in the image below to measure the device in its intended configuration, not only the LED by itself. It is best to have a reference LED and to test at the closest realistic viewing distance. A repeatable measurement provides useful reassurance that the setup is behaving as intended.
In the following image, I evaluate the pupil labs camera against the GC0308 camera by holding both cameras directly over the meter sensor within one minute over each other at the same distance.
Infrared LEDs are useful, efficient tools for eye tracking and many other applications. Choosing a modest-output source, using sensible distance and diffusion, and checking the complete system against a known source with an irradiance meter can make an IR setup both practical and reassuring to use.
This page is educational and does not replace a formal safety assessment or professional advice.