Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 209

developed for digital photo and video cameras. CCDs consist of one piece of n-
p silicon semiconductor material, which is in fact a silicon photodiode cell. The
top layer is divided by lines of electrodes, often of aluminium, and lines of insulator
material, into a raster of many tiny dots called pixels. Today, chips with a size smaller
than 1 cm 2 containing more than 10 megapixels are no exception: a matrix with
more than 3000 3000 dots. The light incident on each pixel area is, as in normal

photodiodes, transformed into an electrical charge. CCDs have no traditional wiring
to all individual pixels. They transport the charge at a pixel across the chip to the edge
of the chip where it is converted into a voltage and read, while memorizing the precise
location from where it originates in the cell's matrix. In CMOS chips, which are made
of metal-oxide silicon, each individual pixel has its own charge-to-voltage conversion
and reading circuit. This reduces the area for light capture and thus the resolution
and sensitivity of the chip. Whereas CCDs require a complicated manufacturing
process, CMOS chips are manufactured in much the same way as most microchips
and can therefore be produced considerably cheaper in a mass-production process.
Today, CMOS technology is rapidly developing to include high-end applications,
thus decreasing the quality gap with CCDs.
A digital photo camera with a CCD or CMOS chip records and remembers the
amount of light arriving from each scene element, which in turn is proportional to the
luminance of that element. With specific software it is now possible to analyse
the complete luminance pattern of that picture. In the case of a picture of a road
scene, the average road-surface luminance and the road-surface luminance unifor-
mities, as well as the luminances of the luminaires are obtained. For absolute values,
appropriate calibration is required. For certain digital photo cameras such specific
software is commercially available. Specific luminance meters that need no separate
calibration, also using CCD or CMOS chips, are being produced as well.

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14.2.1.2 V( ʻ ) Correction

The spectral response of a photocell differs considerably from that of the eye as
defined by the spectral sensitivity V( ) of the CIE Standard Observer, on which all

light units are based. For this reason, a so-called correction filter or filters must be
employed over the cell window in the cell housing. The various makes of cell, and
even cells from the same manufacturer, differ in the degree of correction needed to
match cell to eye. Each cell should therefore be supplied with its own appropriate cor-
rection filter. It should be realised, that exact correction by means of filters is difficult
to achieve and makes meters expensive. Cheaper meters, those with insufficient V(

ʻ

)
correction, need “colour-correction factors” for each different type of light source.
These correction factors have to be supplied by the meter manufacturer. Such meters
are not suitable for the measurement of mixtures of different light sources especially
those including daylight. Given the great number of different spectra that LED light
sources may have, it is impossible for a meter manufacturer to supply correction
factors for all the possible different types of LEDs. Poor V(

ʻ

) corrected light cells
should therefore not be used for the measurement of LEDs or LED installations.

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