Photoemissive Light Detectors ISAT 300 Foundations of Instrumentation and Measurement D. J. Lawrence Spring 1999

Light Measurement (1)  Optical Radiometry is the science of light measurement spanning across the ultraviolet, visible and infrared regions of the electromagnetic spectrum. as it is perceived by the ‘average human eye’  Photometry is the subset of radiometry that deals with the measurement of visible light as it is perceived by the ‘average human eye’.  Optical power and related quantities can be measured in radiometric units or in photometric units.

Light Measurement (2)  Radiometric units are based on the watt (W), which is the fundamental unit of optical power, or flux, in radiometry. Optical power is a function of both the number of photons arriving per second and the energy of those photons.  The lumen (lm) is the photometric analog of the watt and relates to the way the eye of a ‘standard human observer’ responds to light.

Light Measurement (3) Radiometric Units Optical Power = P = Radiant Flux = Energy / Time ~ 1 watt (W) = 1 joule/second (J/s) Irradiance = H = Radiant Flux Density = Power/Area ~ W/m 2

Light Measurement (4)  Most light sources produce light of many different wavelengths. Moreover, the optical power output of a source is generally different at different wavelengths.  Thermal sources, such as incandescent lamps, produce light over a broad spectrum, with the ‘color’ at which the peak of the emission occurs related to the temperature of the filament.  Plasma discharge sources (e.g., xenon or mercury vapor lamps) can produce light at narrow ‘lines’ as well as across a broad band.  Lasers can produce monochromatic light.

Light Measurement (5) as it is perceived by the average human eye  Photometry is the subset of radiometry that deals with the measurement of visible light as it is perceived by the average human eye.  The lumen (lm) is the photometric analog of the watt and relates to the way the eye of a standard human observer responds to light.

Light Measurement (6) Photometric Units  Luminous Flux =   v  = Visible Flux ~ lumen (lm)  Illuminance = E v = Visible Flux Density = “Illumination” = Visible Flux/Area 1 lm/m 2 = 1 lux (lx) =  lm/ft 2 =  foot-candle (fc) 1 fc = 1 lm/ft 2 = lx

Light Measurement (7) Direct Sunlight1-1.3 x 10 5 Overcast Day 10 3 Office 5-20 x 10 2 Standard Candle at 1 meter 1 Full Moon10 -1 Starlight Illuminance Examples (in lux)

Light Measurement (8) Eye Response  Our eyes respond differently to different wavelengths, being most sensitive at 555 nm, in the green portion of the spectrum.  Since photometric units relate to the way our eyes respond to light, whereas radiometric units are used for true power measurements, the conversion from radiometric to photometric units is complicated by the wavelength dependence of our eye response. For example:

Light Measurement (9) Photometric Equivalent of 1 Watt of Optical Power at Different Wavelengths 1 watt  0.27 lm at 400 nm (violet)  25.9 lm at 450 nm (blue)  220 lm at 500 nm (blue-green)  679 lm at 550 nm (green)  683 lm at 555 nm (peak)  430 lm at 600 nm (orange)  73.0 lm at 650 nm (red)  2.78 lm at 700 nm (red)

Electron Energy Photoemissive Light Detectors (1) States Filled with Electrons Empty States “Freedom” Allowed Electron Energy Levels in a Metal

Photoemissive Light Detectors (2)  If a photon has sufficient energy, it can completely remove an electron from a metal.  This is called the Photoelectric Effect and is the principle underlying the operation of phototubes, photomultipliers, and microchannel plates. Empty Free Electron Energy Filled with Electrons Light

Photoemissive Light Detectors (3) -- Phototube  The light-sensitive electrode is called a photocathode.  The photocathode surface is coated with a photoemissive material, i.e., one that releases electrons when struck by light of the wavelength that is to be detected. Examples are Sb-Cs, Sb-K-Cs, and Ag-O-Cs.  The resulting current, called the “photocurrent” is proportional to the number of photons striking the photocathode each second.

Photoemissive Light Detectors (4) -- Photomultiplier Tube  In a photomultiplier tube, the incoming light strikes a photocathode coated with a photoemissive material, just like in an ordinary phototube.  However, the emitted electrons are not immediately drawn to an anode.  Instead, they are accelerated (by a voltage difference) to another electrode, called a dynode.  Each electron that strikes a dynode knocks out several (typically three to ten) electrons. These are called secondary electrons.  Thus, each original emitted electron gets “multiplied”.  There are usually eight to twelve dynodes and the number of electrons gets multiplied at each dynode.  Total multiplication factors of 10 6 or more are commonly achieved.  After multiplication, the electrons are collected by an anode.

Photoemissive Light Detectors (5) -- Photomultiplier Tube  Electrons are indicated by dashed arrows.  For this simple example with n = 3 dynodes, each with a multiplication factor of  2, the overall multiplication factor is  n = 2 3 = 8. Dynodes Photocathode Light Anode + - V battery V out I out +  R load I out = I cathode  n

Photoemissive Light Detectors (6) -- Microchannel Plate  Some night vision imaging systems make use of a device called a microchannel plate, which contains millions of microscopic capillaries.  The inside surfaces of the capillaries are coated so as to act like long dynodes.  This type of night imaging system can intensify both visible and near- infrared light.

Photoemissive Light Detectors (7) -- Microchannel Plate capillaries (not all are shown) microchannel plate inner wall acts like long dynode phosphor-coated anode capillary cross section: photo- cathode hf electron cascade - +