The eye is most sensitive to light in the yellow and green area of the spectrum, specifically 555 nm, and is least sensitive in the blue and red regions of the spectrum. This response is known as the photopic response. The scotopic response is the response of human dark-adapted vision to light. (See figure 1.)

Figure 1. Photopic and scotopic response of the human eye.

Therefore, in order to convert a radiometric measurement into a photometric value it is necessary to correct the spectrum by the photopic curve and multiply the result by 683. The total light output from a light source is called the total radiant flux and is measured in Watts.

The equivalent photometric quantity is known as total luminous flux, and is measured in lumens. Both of these quantities measure the total output spectral power from a light source, but the photometric value is more useful to lighting engineers as it represents what the human eye actually sees. The Photonics Cluster laboratories use a 1 metre diameter integrating sphere to measure this quantity and can measure any source up to 200 mm in diameter. The light is collected by a Bentham spectroradiometer that can measure across the entire visible spectrum. For smaller sources, the Labsphere 25 cm integrating sphere and spectroradiometer are used.

There are three other quantities commonly used in photometry:

• Illuminance (lumens per square metre)
• Luminous intensity (lumens per solid angle, also known as candela)
• Luminance (candela per square metre.)

The photonics Cluster laboratories can measure any of these quantities at a wide range of light levels, by using the Bentham Monochromator. Irradiance (watts per square metre) can also be measured in the UV and IR by using the Ocean Optics HR 2000 spectroradiometer. Together, these spectroradiometers allow the Cluster's labs to measure optical radiation from 190 nm to 1100 nm.

These instruments calculate the photometric values by correcting the radiometric spectrum by the photopic curve and multiplying by 683. This means than these calibrated instruments are repeatable to within 1%. However, the labs also have hand held light meters which incorporate filters that approximate the photopic response. These units are repeatable to about ±2%. This allows the customer to select the option of quick measurements, while making only a small sacrifice in accuracy. The labs use two Konica Minolta instruments, an LS 100 luminance meter and a CL 200 illuminance meter.

The CL 200 also takes colour measurements (see below for explanation of colorimetry.)

Photonics Cluster laboratories also perform scientific measurements of colour. Colorimetric measurements are taken with reference to the tristimulus theory of human vision. This theory states that the human retina has red, blue and green cones. The mixed response of these cones allows the eye to see colour and each type of cone has a colour matching function, known as X, Y and Z respectively. The CIE 1931 diagram uses the tristimulus theory to model colour response in two dimensions.

The advantage of the CIE 1931 diagram is that any colour can be represented by two chromaticity co-ordinates, x and y. The line on the diagram is known as the Planckian locus, and is the region where the different colours mix to produce white light. The position of any white light source on this line is defined by its colour temperature. Low temperatures represent a reddish, "warm" light, high temperatures represent a bluish, "cool" light. Colours that lie close to this line have a correlated colour temperature (CCT.)

The colour rendering index (CRI) defines how well colours are rendered by the measured source of illumination. This index is based on the average of eight standard colour tiles - the smaller the difference in perceived colour, the better the colour rendering. The higher the value of CRI, the closer the source approximates white light.

All three Photonics Cluster spectroradiometers can measure colorimetric quantities to an accuracy of better than 1%. The CL 200 also allows measurement of colorimetric units to a similar accuracy. As well as the hand held meters and the spectroradiometers, the laboratories also have a Radiant Imaging imaging photometric and colorimetric system. This system is a digital camera employing three filters, red, green and blue, enabling images to be taken and immediately analysed for their photometric and colorimetric data.

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