Photometer is a photometric quantity detector; it consists of a silicon photodiode, an aperture, and in some cases a diffuser and V(λ) correction filter. This V(λ) correction filter must follow the total spectral responsivity of the photometer (photodiode, filter, diffuser) to the V(λ) function. Again the photometer head does not need to have cosine correction. It is because this photometer is generally used with an incandescent standard lamp. This lamp is placed on the optical axis of the photometer at a sufficient distance to deliver normal incident light with small divergence angle.
For achieving the accuracy of the photometer responsivity, the reference plane is accurately defined. The reference plane is the plane where the inverse square law is to be defined. So this reference plane is not erroneously defined to break the accuracy of inverse square law based on detection of the characteristics of the photometer.
Here two types of photometer are shown below. Above figure shows that a diffuser is just placed between the aperture and the V(λ) correction filter.
Non Diffuser Type PhotometerIt is a standard photometer without diffuser. It is often used by the national laboratories to realize and maintain the Illuminance unit. By using collimated monochromator output beam this type of photometer is used to measure the spectral responsivity. It exhibits narrow acceptance angle. Narrow acceptance angle is the advantageous for measurement as it blocks the stray light from the ambient. But disadvantage is that it uses the large size lamp at shorter distance.
Diffuser Type PhotometerIt is a standard photometer equipped with a diffuser. It is very commonly used. This diffuser is used for cosine correction. The diffuser provides flat reference plane precisely. Illuminance meter having a dome-shaped diffuser is not adequate for standard photometers. This diffuser is not subjected to the UV ray to be degraded. It exhibits large acceptance angle. But it is less subjected to the errors due to the large sized lamp at shorter distance. It is more subjected to the stray light effects due the large acceptance angle.
The Characteristics of a Good Photometer
- Relative spectral responsivity must be matched with V(λ) function. Generally most of the photometer responsivity does not match the V(λ) function. But good photometer needs to be with very small error.
- Temperature dependence is a factor of the photometer responsivity. Because due to temperature change the photodiode changes its responsivity. So temperature correction procedure should be followed.
- Linearity of the photometer sensitivity depends on the silicon photo diode sensitivity. So high quality photo diode must be provided with the photometer.
- Long term stability should be fulfilled by the photometer. The response of the photometer should not be changed within a short period of time.
- Calibration should be provided with respect to the reference plane. This calibration must be done at 25oC i.e. room temperature. The distance, room temperature and the reference plane should be recorded.
- Reduction of the stray light must be done with care to minimize the error.
- High Illuminance level must be maintained during measurement.
- Maintenance should be taken periodically to make the photometer safe from dust and moisture.