The SRS8 and SRS12 are uniform sources calibrated in spectral radiance for routine calibration of spectroradiometers, telephotometers and luminance meters. Both sources are based on Ba2SO4 coated integrating spheres fitted with a baffled quartz halogen lamp to provide excellent illumination uniformity, and are easily mountable on all optical bench systems or flat surfaces.

Whilst the standard calibration range is 380nm-800nm, this can be extended as defined in the options table below. This 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 200mm diameter SRS8, fitted with a baffled 50W quartz halogen lamp provides an ouput, fitted with a diffuser window, of 50mm diameter. The SRS8 requires a precision constant current dc power supply at 4.000A. A small diameter aperture can be fitted to the window of the SRS8 (option RI), supplied with a spectral radiant intensity calibration. 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.

The 300mm diameter SRS12, fitted with a baffled 100W quartz halogen lamp provides an output of 100mm diameter. The SRS12 requires a precision constant current dc power supply at 8.5A power supply. The SRS12 has been designed to complement the Bentham TEL309 for the measurement of the Photobiological Safety of Lamps according to EN/IEC62471, where, in certain instances, the convolution of a relatively wide field of view and a measurement distance of several metres can lead to the requirement of a large uniform source for the purposes of calibration.

Bentham offers a 250W rated constant current p.s.u. for use with calibration lamps, model 605.

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 telescope.

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.

 

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, see Fig.2.

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.