Light
1.3.1 Light
The velocity of light waves in a vacuum is c = 2.99792458 × 108 ms−1
For any type of wave, including light waves:
For visible light, the frequency is of the order of 1015 Hz. Photodetectors cannot respond to such rapid changes and thus generally indicate rms or mean values of power of the radiation.
The energy of a photon depends on the frequency but is in the order of 10−20 J. Most detectors respond to large numbers of photons but there are a few that can provide a useful output for just one photon.
There is a difference between the measurement of radiant energy and that of light. The term light implies a human connection, and the measurement of visible light is called photometry. The human eye is a very common photometric detector.
Radiometry is concerned with the measurement of radiant energy independent of the type of detector used. A third field of radiation measurement is concerned with the quantum nature of light and is called actinometry – which arose from a study of the photochemical effects of light.
1.3.3 Standards of measurement
The official SI base unit for measuring the luminous intensity of light is the candela. The candela is the only SI base unit which has its origins in the response of a human organ (i.e. the eye) – it is a photometric quantity. The candela is a base SI unit upon which lumens and lux are derived.
The candela has become an important base unit due to the historical nature of measurements of light which involved the human eye as the detector. Early standards by which the response of a human eye were quantified involved candles, flames, and incandescent lamps. Human observers compared an unknown light source to a standard.
Modern methods utilise the response of a device (e.g. a photocell) which has spectral characteristics which are very close to that of a standard observer. Standard sources provide a way of calibrating photocells to be used in industry.
For precise photometric work, it is usually preferable to operate lamps on DC. It is preferable to set the operating current. Measurement of luminous flux may be made by comparison with luminous flux standards using an integrating sphere, or a goniophotometer.
The most commonly used measurement of light intensity is not actually the candela, but the illuminance or brightness and is typically given in lux. In a normal lecture room, the illuminance is about 300 lux. A bright summer’s day: 20000 lux. In daylight, 680 lux corresponds to a radiant flux of about 1 Wm−2
1.3.4 Thermal detectors
In thermal detectors, incoming radiation results in a change of temperature of the sensor. The temperature of sensor is an indication of the magnitude of incident radiation.
Temperature is usually measured with a thermopile, which consists of a large number of thermocouples in series. The sensitivity of a thermal detector using a thermopile with a surface area of 1–10 mm2 is typically about 10–100 V/W, with a time constant of about 10 ms.
The sensitive region of the detector is usually blackened so as to absorb the maximum amount of incoming radiation.
Still yet another type of thermal detector utilises the pyroelectric property of certain ferroelectric materials. Incident radiation causes a change in the surface charge of a residually polarised ceramic. The effect can only be measured in a pulsed mode of operation and hence an AC amplifier is used to produce a reasonable output.
1.3.5 Light dependent resistor (LDR)
In a semiconductor, photoconductivity is a result of an increase in electrical conductivity due to impingement of photons on the semiconductor material. This increase can only occur if the incident photons have an energy hv > Eg where Eg is the energy gap between the valence and conduction bands.
Incident photons cause electrons in the valence band to be given energy hv and, if hv > Eg, valence electrons enter the conduction band, leading to an increase in the number of mobile electrons.
The increase in conductivity manifests itself as an increase in the current through the device for a given applied voltage and as such may be called a light dependent resistor (LDR).
When an LDR is illuminated with a steady beam, an equilibrium is reached where the decay of electrons is matched by the excitation.
The ratio of the number of excited electrons to the number of incoming photons is called the quantum efficiency and is dependent on the probability of the number of elastic collisions between photons and electrons.
For a given frequency of incident beam, the number of mobile electrons created is a function of intensity of the beam. However, the conductivity of the material depends not only on the intensity of the incident radiation, but also upon its frequency. This is due to the filling of available quantum states.
1.3.6 Photodiode
A photodiode employs the photovoltaic effect to produce an electric current which is a measure of the intensity of incident radiation.
1. Near the junction, concentration gradient causes free electrons from n side to diffuse across junction to p side and holes from p side to diffuse across to n side.
2. Resulting build-up of negative charge on p side and positive charge on n side establishes an increasing electric field Ed across the junction.
Current will flow in external circuit as long as photons of sufficient energy strike the material in the depletion region.
The area near the junction becomes free of majority carriers and is called the depletion region. When a photon creates an electron-hole pair in the depletion region, the resulting free electron is swept across the junction towards the n side (opposite direction of Ed).
Even though the photodiode generates a signal in the absence of any external power supply, it is usually operated with a small reverse bias voltage. The incident photons thus cause an increase in the reverse bias leakage current Io.
The reverse bias current is directly proportional to the luminous intensity. Sensitivity is in the order of 0.5 A/W.
1.3.7 Other semiconductor photodetectors
A charge coupled device CCD is an array of closely spaced photodiodes. Incident light is converted to an electric charge in each diode. A sequence of clock pulses transfers the accumulated charge to a digital output stream. For video applications, an image must be focussed on the device using a lens.
1.3.9 Photomultiplier
One of the most common applications of photomultipliers is for the detection of nuclear radiation. But, the device may be also used as the basis for detection of a wide range of phenomena which involve very low light output levels (e.g. chemi-luminescent gas detector). The light sensitive surface of a photomultiplier consists of a thin film of an alkali metal which has a low work function W. When a photon with energy E impinges on the metal, if E > W, then electrons are emitted from the metal. These electrons are accelerated by an applied potential (of about 200 V) towards a dynode. An accelerating electron, when it strikes the dynode, has sufficient kinetic energy to eject two or more electrons from the dynode material.
These two electrons are then accelerated through another 200 V potential to another dynode and thus cause four electrons to be ejected. This amplification may involve several stages of dynodes, each at a potential of 100–200 V above the previous stage. Thus, the final electron current is sufficiently high to measure with conventional electronic equipment.
Amplification is thus done within the evacuated structure and may be as high as 106. Further, this amplification is done prior to the intrumentation amplifier input resistance and noise in the signal is thus reduced considerably. Dark current (due to thermionic emission at cathode) limits detectivity.
Note: A high voltage power supply is needed to produce the required accelerating potentials at each dynode.
1.3.10 Review questions
2. A 100 W motor cycle headlamp can just be seen by a pedestrian two kilometres away. The size of the pupil in the pedestrian’s eye is 1 mm2. Calculate the minimum incident power detectable by the retina of the eye. Assume that the headlamp is 25% efficient in converting electrical energy into visible light.
3. Discuss the differences between the radiometric and photometric definitions of light. That is, why are they different?
4. A photodiode has a sensitivity of 9 nA/lux at 560 nm and an area of 40 mm2. Express the detectivity in A/W. Note: 1 lux = l lumen/m2. A radiant flux of 1 W at 560 nm produces 685 lumens.