Measurement of Light: Applications

ISAT 300

Foundations of Instrumentation and Measurement

D. J. Lawrence

Spring 1999

Light Detection and Measurement : Applications (1)

Light detectors have a wide variety of applications. Detect the presence or absence of light. Quantify the amount of light present. Quantify the fraction of light reflected from or

transmitted through an object. Measure light intensity as a function of

wavelength. 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 ImagingCameras [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 = 16.8 106 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 = 3 108 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 = 6.626 10-34 J-s

= 4.136 10-15 eV-s

E = (h c) / o

h c = 1240 eV-nm = 1.24 eV-m

1 eV = 1.602 10-19 J

= h / 2 = 1.055 10-34 J-s

Properties of Light (3)

Color o (nm) f (Hz) Ephoton (eV)

red 630-760 ~4.5 x 1014 ~1.9orange 590-630 ~4.9 x 1014 ~2.0yellow 560-590 ~5.2 x 1014 ~2.15green 500-560 ~5.7 x 1014 ~2.35blue 450-500 ~6.3 x 1014 ~2.6violet 380-450 ~7.1 x 1014 ~2.9

For the visible portion of the electromagnetic spectrum, the wavelength in vacuum (or in air) ranges from:

o 400 nm 700 nm4000 7000 Åto

to

Åo o o

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).