Radiometry and Photometry

The values of optical radiation are usually measured in radiometric units as watt seconds or watts, However, the visible part of optical radiation has its own photometric units with a basic unit known as candela (cd). It is the sixth unit apart from the basic units length (m), mass (kg), time (s), electrical current (A) and temperature (T), which were defined during the 10th International Conference for Weights and Measures in 1954. All these units have an official measure which is standardised worldwide by calibrating institutes. The length, for example, was defined for a long time by the primary standard meter in Paris, now it is defined by how long it takes the light to pass through 1 meter in a vacuum multiplied by the fixed speed of light in a vacuum. The standards for mass, time, etc. are also well known. On the other hand, the units in light engineering are not as common to us since we do not come across the measurement in our daily lives. These units, however, are indispensable in the preparation of lighting equipment for the various kinds of places people inhabit. In optical science the whole optical, spectral range of radiation is taken into consideration which covers a wavelength range of 0.2 mm to approx. 1 mm. In light engineering or photometry, however, only the range that can be perceived by the human eye is of interest. Since the sensitivity of the eye is a subjective unit, the International Commission for Lighting has defined the curve of the spectral response of the human eye V(λ). Several measurements were taken on people to obtain a statistical result for this purpose. Photometry considers the subjective spectral response (sensitivity) of the human eye combined with strict physical rules. The sensitivity of the human eye is the measure of all things in photometry. Within this experiment the basics of light are given in the comprehensive manual. The black body radiation is discussed in detail as well as the transition to photometric units. The practical part covers the calibration of a standard lamp with certified parameters. For this purpose, the so-called lock in technique in connection with a light chopper and wavelength independent photo detector is used. In addition, the 1/r2 law can be demonstrated and measured.
 

Principle of operation

The heart of the set-up, besides the lock-in amplifier (F), is the standard lamp model WI40/G (A) calebrated in units of luminous intensity. The photo detector (D), has a constant spectral sensitivity in a range of 0.2 - 20 µm and is calibrated in radiometric units. In order to prevent unwanted ambient light to pass to the detector a shield (B) with adjustable iris is used. The solid angle of the light bundle for the measurement is defined by the adjustable diameter of the iris. An almost complete suppression of ambient light is achieved by using the so called lock - in technique. For this purpose the light chopper (C) modulates the light bundle to be measured with a defined frequency, which is determined by the revolution speed and the number of sectors of the chopper blade. The lock - in amplifier receives its gating signal from the controller (CHS) in such a way that a measurement is taken only when the chopper blade is opened, thus obtaining a high degree of suppression of ambient light.