Quote:
Originally Posted by newday07
How many LEDs are there in an ereader? Did you consider the exposure time? |
For the only IR touch screen I dismantled, the number of LEDs did not matter since only two were on at any one time. It used a matrix array in which the LEDs were not on continuously which increases the LED lifespan while reducing the power consumption (the latter being very important to an ereader). The pattern seemed to be turning on a vertical LED and then scanning by sequentially pulsing the horizontal LEDs. Move to the next vertical LED and loop until an event is seen.
The other issue you seem to be disregarding is that the IR LEDs in a touch screen are rated in milliWatts (the one I dismantled used Osram 940nm 50mW LEDs as the senders), the incandescent lamp is rated in Watts while Ghod alone knows what the wattage equivalent for the sun at ground level would be -- NASA uses 1360 Watts per square meter for the top of atmosphere but atmosphere and sun height in the sky make a ground level number hard to determine.
To quote from one study, "The threshold i.r. irradiances for cataract formation were determined from the relationship between the incident irradiance and the lens temperature, and were in the range 163-178 mW cm-2 for long-term exposures (greater than about 5 min) under normal conditions." This was for workers in industries such as steel making.
One other study using rabbits used a 250W infrared reflector lamp (what most people call a heat lamp) placed 20cm from the rabbit's eyes. Hmmm.... 60mW not aimed at the eyes, 250W aimed directly at the eye. Hold your hand in front of your ereader and feel the heat -- not!
For the exposure time, if the level of IR is too low to significantly heat your eyes, it's too low to do any damage regardless of length of exposure. IR is just not as energetic as UV.
So basically, if you can't feel the heat, it's not going to hurt your eyes.
Please note that this is not suggesting using your remaining eye to look at an IR laser diode.