Measurement of Light: Applications ISAT 300 Foundations of Instrumentation and Measurement D. J. Lawrence Spring 2000

Light Detection and Measurement : Applications (1) K Light detectors have a wide variety of applications. K Detect the presence or absence of light. K Quantify the amount of light present. K Quantify the fraction of light reflected from or transmitted through an object. K Measure light intensity as a function of wavelength. K Optical imaging, e.g., cameras.

Light Detection and Measurement : Applications (2) Applications Requiring Detection of the Presence or Absence of Light:  Optical Communications (digital)  CD and DVD Players  Optical Data Storage Systems (disk and tape)  Bar Code Readers  Displacement Sensors (e.g., photogates for measuring linear motion and encoders for measuring the rotation of a shaft)  Optical Range Finders  Fluorescence-Activated Cell Sorter

Light Detection and Measurement : Applications (3) Applications Requiring Quantitative Light Measurement:  Radiometers/Photometers (used to measure light output provided by some source, for a wide variety of studies, e.g., solar energy, plant growth, lighting)  Spectroradiometers (used to measure light output provided by some source as a function of wavelength)  Spectrophotometers (used to measure the fraction of light that is reflected from or transmitted through an object as a function of wavelength; used for a wide variety of studies, e.g., characterization of properties of windows for solar collectors, and determination of ion concentration in solutions)

Light Detection and Measurement : Applications (4) Applications Requiring Optical Imaging  Cameras [wide variety of types, including conventional silver halide film, vidicons for TV, and charge-coupled device (CCD) solid state cameras]  Low-Light Imaging Systems (e.g., image intensifiers and night vision systems)

Light Detection and Measurement : Applications (5)  Incremental digital displacement sensors. Rotary encoder (transmission type): (a) front view and (b) side view. Linear encoder: (c) transmission type and (d) reflection type.

Light Detection and Measurement : Applications (6)  The major components of a cell sorter are shown. Some cells have been “tagged” with a fluorescent dye, so that when the laser beam hits them, they emit light of a different wavelength. This “fluorescence” is detected by a photodiode or a photomultiplier tube (PMT) and the resulting electrical signal is used to determine which cell collector each cell goes to. (graphics courtesy Hamamatsu Photonics)

Light Detection and Measurement : Applications (7)  The detector ring of a positron computed tomography (positron CT, or positron emission tomography, or PET) system is shown. A positron- emitting radioactive isotope is introduced into the body. When the positrons are annihilated by combining with electrons in the body, gamma ray photons are emitted. These are detected by the scintillator- PMT assemblies. This system can create images mapping blood flow into the brain. (graphics courtesy Hamamatsu Photonics)

Light Detection and Measurement : Applications (8)  Two turbidimeter geometries are shown. These instruments measure the “cloudiness” or “haziness” of a liquid caused by suspended particles. (graphics courtesy Hamamatsu Photonics)

Light Detection and Measurement : Applications (9)  A square array of InSb photodiodes is shown. The array consists 128  128 (= 16384) photodiodes. The individual photodiodes are too small to be seen in this figure. Since these photodiodes can detect wavelengths up to 5400 nm = 5.4  m, such an array can be used as an infrared-sensitive camera for night vision. Arrays of light detectors (photodiodes, photoconductors, or photocapacitors) can be made from a variety of materials. Silicon camera arrays consisting of 4096  4096 = 16.8  10 6 detectors, or more, are available.

Light Detection and Measurement : Applications (10)  The simplified block diagram of a transmission spectrophotometer is shown. The “monochromator” uses a prism, or more often a diffraction grating, to break “white” light from the lamp into its constituent wavelengths. Only one wavelength at a time is allowed to strike the sample. The fraction of the light that is transmitted through the sample is recorded. This quantity is called the transmittance and, in general, it depends on the wavelength. (graphics courtesy Hamamatsu Photonics)

Properties of Light (1) f = frequency (Hz) o = wavelength in vacuum or air [usually measured in  m, nm, or Angstroms (Å)] c = speed of light in vacuum = 3  10 8 m/s c = f o n = refractive index of a material (“medium”) v = c / n = speed of light in material = o / n = wavelength in material v = f

Properties of Light (2) E = h f = energy of a photon h = Planck’s constant =  J-s =  eV-s E = (h c) / o h c = 1240 eV-nm = 1.24 eV-  m 1 eV =  J  = h / 2  =  J-s

Properties of Light (3) Color o (nm) f (Hz)E photon (eV) red ~4.5 x ~1.9 orange ~4.9 x ~2.0 yellow ~5.2 x ~2.15 green ~5.7 x ~2.35 blue ~6.3 x ~2.6 violet ~7.1 x ~2.9 For the visible portion of the electromagnetic spectrum, the wavelength in vacuum (or in air) ranges from: 400 nm 700 nm Å to Å

Properties of Light (4)  Light with wavelength o  < 400 nm is called ultraviolet (UV).  Light with wavelength o  > 700 nm is called infrared (IR).  We cannot see light of these wavelengths, however, we can sense it in other ways, e.g., through its heating effects (IR) and its tendency to cause sunburn (UV).

Photoemissive Light Detectors

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.3

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 ( sometimes designated as I )

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 Source Output vs Wavelength

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)

Eye Sensitivity vs Wavelength 100

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

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 - +

Semiconductor Light Detectors

Photodiodes - Photodiodes are semiconducting devices that convert light into electrical signals. - There are several kinds of semiconductor photodiodes. They all work on the same principle which is based on photoconductivityphotoconductivity. - Can tailor the response to desired wavelengths by choosing materials with the desired band gap eV value. - Photodiodes have invaded our homes: they can be found in appliances that can be controlled by remote control.

Example Detector Radiometric Responsivity 6.2 eV4.1 eV3.1 eV2.5 eV2.1 eV1.8 eV1.6 eV1.4 eV1.2 eV