Green light needs little power to appear visible.
The illuminance of light, measured in lux, describes how the eye perceives its intensity. The light's strength, measured in milliwatts, determines this value. Yet illuminance also depends on the human eye's variable sensitivity to light. The eye is most sensitive to green light, which has a wavelength of 555 nanometers. Other forms of light on either side of the spectrum need a greater amount of power to reach the same level of illuminance.
Instructions
1. Determine the eye's relative sensitivity to the light. For example, if you are calculating the illuminance of 640 nm light, you will see that the eye's relative sensitivity to it is 0.175. See Resources for the Rensselaer Polytechnic Institute link.
2. Multiply the light's power, measured in milliwatts, by this sensitivity. For example, if the light has a power of 500 mW: 500 --- 0.175 = 87.5.
3. Multiply this answer by 683, a conversion constant: 87.5 --- 683 = 59,763. This is the light's illuminance, measured in lux.
Tags: illuminance light, light power, measured milliwatts, relative sensitivity, sensitivity light