The v(lambda) Curve

The sensitivity of the eye is not constant over the whole electromagnetic spectrum. To start with, the human eye is only sensitive to the wavelengths between 380 and 780nm. We call this the 'visible spectrum' or 'light'.

Even over the visible range, our eye reacts differently to different wavelengths. Its sensitivity is zero at either end of the visible spectrum and peaks at 555 and 507nm for daytime (photopic) and nighttime (scotopic) vision respectively.

That's why it is not a simple task to measure light. The characteristic of the instrument used needs to be matched to the one of the human eye. This is done by applying a so called v(lambda) correction. It results in the instrument being most sensitive at 555nm and less responsive towards the lower and the upper end of the visible spectrum, just like our eyes.

The daytime curve, also known as v(lambda) curve, is applicable for luminances above 30cd/m². Nighttime vision relates to the v’(lambda) curve and is valid for luminances below 0.003cd/m². Between these two values, a mix of the two spectral response curves occurs.

Photopic and scotopic vision

Please note that for skotopic vision, no colours can be distinguished. All objects will look grey.