ected by the eye, i.e. that radiation which falls between the where a is the wavelength (m), f is the frequency (Hz) and c is the wavelengths 380nm and 750nm. The basic unit of power in photo- speed of light in a vacuum (approximately 2.99 X metry is the lumen (Im). In radiometric measurements the ideal detector is one which has (7.1) a flat response with wavelength whereas in photometry the ideal detector has a spectral response which approximates to that of the average human eye. To obtain consistent measurement techniques The preferred unit of length for specifying a particular wavelength the response of the average human eye was established by the in the visible spectrum is the nanometre (nm). Other units are also Commission Internationale de l'Eclairage (CIE) in 1924. The re- sponse known as the photopic eye response is shown in Figure 7.2 in common use, namely the langstrom ) and the micrometre or and is observed to peak in the green/yellow part of the visible micron. The relation of these units is as follows: spectrum at 555nm. The curve indicates that it takes approximately 1. 1 nanometre (nm) = 10-superscript(0) metre. ten times as many units of blue light as green light to produce the same visibility effect on the average human eye. 1 angstrom (A') = 10-10 metre. The broken curve in Figure 7.2 with a peak at 507nm is termed the 3. 1 micron 10-6 metre. scotopic eye response. The existence of the two responses arises out of the fact that the eye's spectral response shifts at very low light The micron tends to be used for describing wavelengths in the levels. (See Section 7.8.2.) infrared region and the nanometre for the ultraviolet and visible In normal circumstances photometric measurements are based on regions. the CIE photopic response and all photometric instruments must have sensors which match this response. At the peak wavelength of 555nm of the photopic response one watt of radiant power is defined as the equivalent of 680 lumens of luminous power. In order to convert a radiometric power measureme