The SRS8 is a uniform source calibrated in spectral radiance designed for routine calibration of spectroradiometers, telephotometers, luminance meters, etc. It comprises a 200mm diameter integrating sphere with a 50mm diameter window (diffuser) and is easily mountable on all optical bench systems or flat surfaces.

The calibration is performed with respect to National Physical Laboratory (NPL Teddington UK) calibrated lamps held by Bentham. Alternatively, direct NPL calibration can be offered.

The SRS8 is fitted with a 50W quartz halogen lamp. It requires a precision constant current dc power supply at 4.000A.

A small diameter aperture can be fitted to the window (option RI) and is supplied with spectral radiant intensity calibration.

Standard calibration range is 380nm-800nm, but this can be extended to cover 300nm to 2500nm as defined in the options table below.

Other options include quartz diffuser window in lieu of glass and a filter holder to accomodate a range of neutral density and colour temperature shifting filters.

Bentham offers 2 variants of a 250W rated constant current p.s.u. for use with calibration lamps, models 605 and 607. The latter includes a current ramping facility for lamp switch on and off.

Spectral radiance is probably the most common spectroradiometric measurement, especially in the visible region and is most easily visualised as surface brightness. It is useful for characterising displays.

In order to measure spectral radiance which has the units W. m-2 sr-1 nm-1 we need to be able to measure the flux emitted per nm into a known solid angle by a known area of the source.

Fig.1 shows how this can be achieved using the TEL301 telescopes

The solid angle w is set by the diameter of the telescope lens and its distance to the source. The area of the screen from which light is measured is determined by the size of the aperture used in the telescope. For radiance measurement it is essential that the element of the screen actually measured is determined only by the aperture. In practice this is easily achieved by ensuring that the image of the source produced at the aperture overfills the aperture.

The system is calibrated by measuring from a source of known spectral radiance (Bentham SRS8) using the selected aperture. Once again it is essential that the image of the source overfills the aperture.

Accurate measurement of luminance from most sources also requires control of the viewing direction. For lambertian sources (e.g. SRS8) the viewing direction does not matter as the area viewed increases at the same rate as the output in the view direction decreases. (The same effect makes the full moon appear as a flat disc).

Most display devices, however, are not lambertian and show a variation in output as a function of viewing direction which is much greater than that due to the cosine law.

In these cases, it is important to ensure that a constant (usually normal) view direction is used.

Fig.1.

 

The Spectral Radiant Intensity parameter only has significance for sources that are viewed from a sufficiently large distance compared with their maximum dimension that they appear as a single point of light with no discernible shape. The units are W sr^-1 nm^-1.

The most common example is LED indicators.

For this measurement we need to measure the flux emitted from the whole of the source into a known solid angle.

This can be achieved by selecting an aperture for the TEL301 such that the entire image of source passes through the aperture.

It will be noted that in this measurement there are no compensating effects occurring as the lens-to-source distance is changed. It is essential therefore that calibration and measurement distance are either kept the same or that a correction is made according to the inverse square of the distance.

Fig.2.