ActiveTwo Photocell Sensor With Human-Eye-Like Light and Color Sensitivity (price includes required amplifier/converter channels). This photodiode has a 1 cm2 active area, BNC package detector-filter combination which duplicates the response of the most commonly available optical aid; the human eye. The eye senses both brightness and color, with response varying as a function of the wavelength. This response curve is commonly known as the CIE curve. The Ap filters accurately match the CIE curve to within 8% of the area.