Gamma and LEDs – Adjusting for Brightness

If you’ve ever dimmed a programmable led, you have certainly noticed that the response is non-linear. If you ramp up the duty cycle from zero to one hundred percent, you’ve noticed that the led gets brighter very quickly at the start, then somewhere around the fifty percent mark it stops getting brighter at all. On the Pixent LEDs, stepping from a green value of 5 to a green value of 10 more than doubles the apparent brightness; while stepping from a value of 250 to 255 is barely noticeable. It’s not the LED that’s to blame, however. LEDs will produce luminous light output in a very linear fashion as more current is supplied to them. What’s to blame is your eyes and the way they perceive light. Humans perceive brightness using a power law. If (B) is perceived brightness and (L) is the luminance, the amount of physical light that’s getting through your irises, the relationship looks like this:

This exponential relationship, requiring more and more additional light to create a perceptible difference in brightness, is characterized by the Greek exponent: gamma. Arbitrarily picking gamma of 2 makes that fractional gamma exponent into a more comfortable square root and usually isn’t too far wrong. 2.2 is the standard middle value for flat screen monitors. But if you want to go the extra mile, you’ll need to tweak the gamma to your particular ambient light conditions. If you gamma correct with a value that’s higher than your eye’s natural gamma an image will be too contrasty – there will be jumps in the brightness where you would want it to be smooth. If the gamma is set lower than you’re eye’s gamma, differences will be muted and it will look muddy. Get it just right, and you will get a smooth transition from dark to bright across the full range of the LED.

If you’re interested in learning more, drop us a line at info@summitech.com. We can help you to get the most out of your programmable LEDs and LED strips.

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