Photo Sensor

Photo sensor are classed by the physical quantity that is affected by the light,  and  the  main  classes  are  photo resistors,  photovoltaic  materials,  and photo emitters.  Historically,  photo emitters  have  been  more  important  in unraveling  the  theory  of  the  effect  of  light  on  materials,  but  photo voltaic materials, notably selenium, were in use for some considerable time before the  use  of  photoemission  became  practicable.  Since  photoemission  allows us  to  combine  the  description  of  a  usable  device  with  the  quantum  effect, we will consider this type of sensor first, which can also be used to a limited extent  as  a  transducer.  The photo emissive  cell,  in  fact,  was  the  dominant type of photo sensor for many years, and played a vital part in the development  of  cinema  sound  since  it  was  the  transducer  first  used  for  converting the film soundtrack into audio-electrical signals.

A simple type of photo emissive cell is illustrated in figure 1.  This is a vacuum  device,  because  the  photo emissive  material  is  one  that  oxidizes instantly and violently in contact with air, and even in quite low pressures it will oxidize sufficiently to prevent any photoemission. The photo emissive action  is  the  release  of  electrons  into  the  surrounding  vacuum  when  the material is struck by light. This release of electrons will take place whether the   electrons   have   anywhere   to   go   or   not,   but   unless   there   is   a   path provided,  all  the  electrons  will  lose  energy  and  return  to  the  emitting surface,  often  termed  the  photocathode.  By  enclosing  another  metal -  a nickel  wire,  for  example –  which  is  at  a  voltage  more  positive  than  the photocathode, the cell can be used as part of a circuit in which current will flow when light strikes the photocathode. When light releases electrons, the current  that  flows  is  proportional  to  the  amount  of  energy  carried  by  the light beam.

Figure 1   A typical photo emissive cell. The cell must be contained in an evacuated enclosure,  and  consists  of  a  nickel  rod  anode  with  a  nickel  sheet  cathode  coated with a photo emissive material such as cesium.

This, however, is true only if the light is of a frequency that will release electrons.  For  any  photo emitting  material  there  will  be  a  threshold frequency  of  light.  Below this threshold frequency, electrons will not be emitted no matter how intense the light happens to be, so that, in general, photo emitters do not respond to infrared, particularly the far infrared. The explanation of this threshold effect is due to Einstein, and makes use of an idea that was earlier put forward by Planck.

Planck’s theory was that energy existed in units just as materials exist in atoms, and he named the unit the quantum. The quantum is a unit of action, a  physical  quantity  that  was  not  considered  of  practical  interest  prior  to Planck’s  theory.  The  size  of  the  quantum  for  a  light  beam  is  equal  to  the frequency  of  the  light  multiplied  by  a  constant  that  we  now  call  Planck’s constant.  We touched on this idea when considering laser interferometry, because   the   factor   that   prevents   light   from   most   sources   from   being coherent is that it is given out in these quantum-sized packets rather than as a truly continuous beam.

Einstein reasoned that the size of the quantum affected the separation of an  electron  from  an  atom  in  a  photo emitter,  and  if  the  amount  of  energy carried  in  one  quantum  was  less  than  the  amount  of  energy  needed  to separate   an   electron,   then   no   separation   would   take   place.   The   total amount of energy carried by the beam was of no importance if the units of energy were insufficient to separate the electrons. The theory was confirmed by experiment, and this work also established Planck’s quantum theory as one of the main supports of modern physics.

The practical effect as far as we are concerned is that photo emissive cells have a limited range of response, and whereas it is easy to make cells that sense ultraviolet light (high frequency), it is much more difficult to prepare materials  that  will  sense  infrared.  Most  photocells  are  noticeably  much more  sensitive  to  light  in  the  blue/violet  end  of  the  spectrum  than  in  the orange/red end.

Figure 2   The use of a photo emissive cell in a circuit. The supply voltage is in the range  +25 V  to  +300 V  with  a  load  resistor  to  provide  a  signal  voltage  to  the amplifier.

Mixed photocathode materials containing antimony along with the alkali metals cesium, potassium and sodium, have been the most successful emitters in terms of providing an electrical output that is reasonably well maintained for the visible frequencies of light.  This does not imply, however, that the output is by any means uniform.

The photo emissive cell is used in a circuit of the type shown in Figure 2, with  a  voltage  supply  that  is  often  in  the  region  of  25-100 V  between anode and cathode. Some circuits make use of the current through the cell directly,   but the most   common circuit is as shown here, using   a load resistor in series with the cell and amplifying the voltage signal. This is particularly useful when the incoming light is modulated in some way so that the  electrical  output will  be  an  AC  signal, such  as  that  in  use as  a cinema soundtrack transducer.

Where a DC output is needed, the use of a photo emitter is less simple, and a   current   amplifier   is   more   useful   than   a   load   resistor   and   voltage amplifier. The current through a typical cell is of the order of a micro amp, so that fairly large load resistors and a considerable amount of amplification will be needed.  This  creates  difficulties  with  both  frequency  response  and noise  level  when  the  photocell  is  used  to  convert  modulated  light  signals into AC signals.

For measuring purposes, any photo emitter detector will have to be calibrated  for  each  light  color  for  which  it  will  be  used,  and  if  a  reasonably high precision is needed, this calibration will be a long and tedious matter. It is important to realize that `white’ light is a mixture of all the frequencies of the visible spectrum, and can also contain a proportion of invisible ultraviolet. This means that imperceptible changes in the composition of `white’

light  which  have  no  effect  on  its  total  energy  will  nevertheless  have  very large  effects  on  the  output  from  a  photo emissive  cell.  This  is,  in  fact, something  that  affects  most  photosensitive  devices,  as  keen  photographers will know.

Figure 3   (a) A photomultiplier using three dynodes (or secondary multiplying electrodes). The electrons emitted from the transparent photocathode are multiplied in  the  dynode  stages  to  provide  a  much  greater  output  current  than  would  be obtained from the original photocurrent. (b) The secondary emission characteristic for a typical dynode material.  Materials that are good photo emitters are usually good secondary emitters as well.

Comparisons of light levels must therefore be made only when the composition  of  light  is  constant,  and  this  is  something  that  is  very  difficult  to achieve, particularly for natural lighting. For artificial lighting using fluorescent  tubes,  constant  light  composition  is  much  easier  to  achieve,  but filament  lamps  give  a  light  that  contains  a  large  fraction  of  red  light  and for   which   the   light   composition   varies   very   sharply   as   the   voltage   is altered.  A  filament  lamp  which  is  run  below  its  rated  voltage  gives  light that  is  predominantly  red;  when  run  above  its  rated  voltage  it  can  give light that is biased to the blue end of the spectrum (with a greatly reduced life).

Photo emitters are still used where a fast response is needed, because many competing  devices  are  solid-state  rather  than  high-vacuum,  and  the  speed of  electrons  in  a  solid  is  very  much  lower  than  the  speeds  that  can  be obtained in a vacuum. Even for cinema soundtracks, however, the vacuum photocell  has  now  been  replaced,  and  at  the  time  of  writing,  vacuum devices  are  found  only  in  old  equipment  and  in  instruments  intended  for specialized   use.   For   these   reasons,   then,   more   detailed   descriptions   of vacuum photo emissive cells will not be given here. However, the photomultiplier  obtains  very  much  greater  sensitivity  from  a  photocell  at very  little cost in noise or time delay.

The  principle,  illustrated  in  Figure  3,  makes  use  of  secondary  emission from  the  same  type  of  materials  as  are  used  for  photo cathodes,  notably cesium.

Figure 4   The electrical arrangement for the photomultiplier, using a chain of resistors (1-12 MO values) to supply the dynodes.

Secondary emission occurs when a material is struck by electrons and releases more electrons than strike the surface initially. The effect is at its peak for electrons that have been accelerated by a voltage in the range of  30-200 V,  and  for  surfaces  of  cesium  the  multiplication  factor  can  be large, for example, 3-7. This means that an electron beam from a cathode  can  be  directed  to  a  secondary  emitting  surface  and  be  re-emitted  as  a beam that contains a much larger number of electrons. In practical terms, this means a beam at a higher current.

By   cascading   stages,   this   multiplication   effect   can   be   very   large,   so that,  for  example,  five  stages  that  each  give  a  multiplication  of  5  will increase   the   current   of   a   beam   from   a   photo emitter   by   a   factor   of 5 x 5 x 5 x 5 x 5 = 3125.  This  is  unique  among  amplification  methods  in being  virtually  noiseless,  and  photomultipliers  are  used  in  detectors  for very  low  light  levels  for  specialized  purposes.  The secondary multiplying electrodes are   known as   dynodes, and   each dynode   must   be   run at   a voltage level that is substantially greater than the one preceding it ^ Figure 4 shows a typical DC supply arrangement.

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