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Absorptance
The dissipation of light (radiation) within a surface or medium, caused
by the conversion of radiant (luminous) energy to a different form of energy,
usually heat, by interaction with matter. The absorption is the "missing piece",
when comparing the total reflected and transmitted energy with the incident
energy. The ratio of the total absorbed radiant or luminous flux to the incident
flux is called absorptance. The standard unit of absorptance is percent (%)
or a factor between 0 an 1. Absorptance can also be inferred from the transmission
through a medium. If the %transmission of a certain wavelength is 70% then
the material has a 30% Absorptance.
Actinic
The characteristic of radiation that indicates its capability to produce
chemical change. In our industry the term is usually used with reference to
UV radiation and its effects on biological systems. Actinic strips are used
in UV processing to monitor the intensity of sources. The color or optical
density of the strip changes with the exposure dosage.
Ambient Light
Ambient light is the light diffused in the environment surrounding a detector
measuring the optical radiation from another source. This light contributes
to the signal measured from the source. To make valid measurements, the contribution
from the ambient light or background must be subtracted from each measurement.
Aperture
A hole through which radiant energy can pass. Angular aperture is the angle
through which the most divergent rays can pass through a hole or lens. The
aperture of a lens is often expressed in terms of an f/#. The f/# is the ratio
of the focal length of the lens to its diameter. A lens with a focal length
of 100mm and a diameter of 25mm would have an aperture of f/4.
Attenuator
A device which reduces the amount of energy reaching a sensor. Attenuators
are typically used when the radiant energy would saturate a detector. The
QNDS,QNDS2 and QNDS3 filters are attenuators that reduce flux density at the
detector by factors of 10, 100 and 1000, respectively.
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Bandwidth
Bandwidth describes the size of a spectral segment. A bandwidth of 10nm
indicates a range of 10nm of radiation. This can be, for example, between
500nm and 510nm, 1000nm and 1010nm or an equal size segment anywhere in the
spectrum.
Band Eliminator Filter
A band eliminator filter allows the wavelengths above and below the filter
cutoff to pass while suppressing the wavelengths within the band. These filters
are also called notch filters. A 500nm band eliminator filter with a band
pass of 10 nm would suppress the wavelengths between 495nm and 505nm.
Beamwidth
The angular width of a cone of light whose apex is at the source. Beamwidth
is usually defined as the angle subtended by a cone encompassing 90% of the
energy.
Black Body
A black body is an object that absorbs all radiant energy that hits it.
When heated a black body emits a well defined characteristic spectrum which
can be used in the characterizing the spectral responsivity of detectors.
As there is no such thing as a perfect black body, black body simulators are
used for this purpose.
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Calibration
The process of normalizing the signal output from a detector to that of
a detector defined as a standard (usually defined by National Institute of
Standards and Technology (NIST) under identical illumination conditions).
Calibration can also be accomplished by the use of a standard source (lamp)
whose output energy at specific wavelengths and measurement distances are
traceable to the standard lamp defined by the governing standards body (NIST).
Candela (cd) photometric measurement
The SI unit of luminous intensity. One candela is equivalent to 1lumen/steradian.
Candle photometric measurement
1candle = 1.02 candela (cd)
Candlepower (cp) photometric measurement
The luminous intensity of a source expressed in candelas. A source with
a luminous intensity of 10 candelas would be said to be a 10candlepower source.
CCD
A CCD (charge coupled device) is a light detector with high sensitivity
primarily in the visible spectrum. CCDs are typically made in either linear
or two-dimensional arrays consisting of up to millions of individual detector
elements. The 2D versions are used for image recording and are found in most
digital camera used in both scientific and consumer applications.
Chromaticity
The aspects of color associated with hue and saturation without reference
to brightness.
Chromaticity (CIE) Coordinates
The proportion of the standard tristimulus values used in color matching.
Colors are compared by their CIE X, Y, and Z coordinates.
CIE
The CIE (Committee Internationale de l’Eclairage) is the international
standards organization for illumination and color vision.
Color Temperature
Color temperature refers to the temperature, in degrees Kelvin, that a
black body would have to be heated to in order to have a color similar to
the reference. A 40W incandescent bulb has a color temperature of about 2680K
while daylight at noon has a color temperature of about 5500K.
Cosine Collector
A cosine collector is a translucent light collector that compensates for
normal blocking of radiation from flat surfaces. The cosine collector samples
the radiation according to the cosine law through the hemisphere above the
surface. A cosine collector may also be referred to as a Lambertian surface.
Cosine Law (Lambert’s cosine law)
The flux per unit solid angle leaving or entering a surface is proportional
to the cosine of the angle with respect to the normal to the surface. In a
cosine collector the rays that strike the surface at an angle of 60° from
the vertical will have a contribution of 0.5 (cosine of 60°) that of identical
rays arriving vertically.
Cutoff Filter
A filter that does not pass light of wavelengths shorter that the cutoff
wavelength and passes a wide band of wavelengths above the cutoff wavelength.
The cutoff wavelength is specified at some point along the transition from
maximum transmission to 0 transmission. The same filter can have different
cutoff wavelengths depending upon the specified % transmission. The same filter
may be specified as a 500nm cutoff filter with the 50% transmission point
as the reference or a 485nm cutoff filter if the specification is the 5% transmission
point, where the transmission at 485nm is 5%.
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Dark Adaptation
The ability of the human eye to adjust itself to low light levels.
Dark Signal (Current)
The dark signal is the signal that flows in a photodetector when there
is no optical radiation impinging upon it. This signal is created within the
detector and the amplification circuits due to thermionic (temperature) effects.
Densitometer
A densitometer measures the opacity or absorptance of a material. The measurement
is usually expressed in AU (absorptance units) or OD (optical density).
Diffraction Grating
A diffraction grating is an optical component that separates light into
its constituent wavelengths. Functionally equivalent to a prism, it disperses
the light into its spectrum by employing grooves to diffract the light. The
angle of the diffraction is a function of the wavelength.
Diffuse Reflectance
The ratio of the incident flux to the reflected flux from a scattering
surface as opposed to a highly directional or specular (mirror-like) surface.
Dose
A common, but loosely used, term for energy density, or radiant flux density,
at a surface. (It is a precisely defined term in EB curing: 1 Gray (Gy) =
1 J/kg , a measure of absorbed energy per unit mass). In other technologies,
the term usually applies to energy absorbed within the medium of interest,
but in UV curing, is equated only to irradiant energy density arriving at
the surface of the medium of interest. [The preferred shortened term is energy
density, expressed in J/cm² or mJ/cm²].
Dynamic Range
The dynamic range is ratio of the maximum measurable signal before saturation
to the minimum measurable signal above the noise. Typically the dynamic range
is expressed in either decades (powers of 10) or bits (power of 2). A 5 decade
dynamic range indicates that there is a factor of 100,000 between the maximum
and minimum signals measurable with the apparatus. A dynamic range of 16 bits
(264) indicates a factor 65,532 between the minimum and maximum measurable
signals.
Dynamic range is also expressed in db (decibels), defined as 10 log10 (maximum
signal / minimum signal). Five decades of dynamic range is equal to 50 db.
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Einstein
A unit of energy equivalent to the amount of energy absorbed by one molecule
of a material undergoing a photochemical reaction as determined by the Stark-Einstein
law.
Electromagnetic Radiation
Radiation emitted by vibrating charged particles. A combined oscillation
of electric and magnetic fields that propagates through space at the speed
of light. The electromagnetic spectrum is theoretically infinite, includes
gamma, x-ray, UV, visible, IR, microwaves, and radio waves.
Emissivity
The ratio of an object’s radiance to that emitted by a black body at the
same temperature and wavelength.
Energy Density
Radiant energy arriving at a surface per unit area, usually expressed in
joules or millijoules per square centimeter (J/cm2 or mJ/cm2). It is the time-integral
of irradiance. (Other terms applied include "radiant exposure," "light dose,"
and "total effective dosage").
Etendue
Also called the throughput of an optical system, it is the product of its
entrance aperture and the solid angle in which light can be accepted at that
aperture.
Exitance
Flux leaving a surface per unit area.
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Flux
The energy per second (power) in a light beam expressed in Watts or Joules/second.
(Radiant power). In photometric measurements the luminous power is typical
expressed in lumens (lm) .
Footcandle (fc) photometric measurement
Footcandles is a unit of measurement of illuminance (how much light is
striking a point on a surface). Once footcandle is equivalent to 10.764 lux.
Footlambert (fl) photometric measurement
A unit of luminance equal to 1/p candela /ft2.
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Germicidal
All biological organisms contain DNA. DNA is essential to reproduction.
Optical radiation in the UVC range is capable of breaking the molecular bonds
within DNA effectively killing microorganisms. Germicidal UV lamps are used
for water treatment, sterilization of foods and their containers, and air
purification, especially in hospital environments.
Hefner Unit photometric measurement
A unit of luminous intensity equal to 0.9 candle.
Illuminance photometric property
Luminous flux incident per unit area of a surface. 1 lumen/m2 = 1lux.
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Infrared (IR)
The invisible portion of the electromagnetic spectrum that extends from
0.75 microns to 1000 microns. Radiation in the near infrared (NIR) produces
the sensation of heat.
Integrating Sphere
A hollow sphere coated with a white diffusing coating in the interior.
It is used for measuring the diffuse reflectance and transmittance of objects
or the total flux from a source that is completely inside.
Intensity
Flux per solid angle. Radiometric measurements are made in W/sr. Photopic
measurements are made in lumens/sv.
Inverse Square Law
The inverse square law correlates the relative intensity at varying distances
from a point source. The relative intensity will diminish to a factor of the
square root of the difference in distance. For example if at 2 meters from
a source the intensity is 16 W/m2, it will be 4W/m2 at 4meters and W/m2 at
8 meters. For extended (non-point) sources the intensity fall off approaches
the inverse square law at a distance equivalent to 5 times the diameter of
the source.
Irradiance radiometric property
Radiant flux incident per unit area of a surface; the power incident per
unit area. The radiometric unit of measure is W/m2 or factors thereof (mW/cm2).
The photometric units of measure are lumens/m2, lux, phot, and footcandles.
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Joule (J)
The Joule is the SI unit of energy.
Lambertian Surface
A surface whose emission or scattering follows Lambert's Cosine Law, in
which the radiant intensity leaving a surface is proportional to the cosine
of the angle from surface normal. See Cosine Collector.
Linearity
The precision with which there is a direct relationship between the incident
radiation and the resultant measurement value up to a point of saturation.
A linearity of 1% states that the ratio of the measured value to that of the
incident radiation will not vary more than 1% from the absolute.
Lumen (lm) photometric measurement
The lumen is the photometric unit of power. It is the flux emitted in a
unit solid angle by a point source that has one candela luminous intensity.
Luminance photometric property
Flux density per unit solid angle.
Lux radiometric measurement
S.I. unit of illuminance equal to 1 lumen per square meter.
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Micron
A unit of length equal to 10-6 m. Infrared wavelengths are typically measured
in microns.
Monochromator
A monochromator is a devise that uses a diffraction grating or prism to
disperse light into a spectrum of its constituent wavelengths. The dispersive
element is rotated such that only a narrow (monochromatic) band of light is
permitted to exit the monochromator through a narrow aperture or slit.
Nano
Prefix denoting 10-9. One nanowatt (nW) = 10-9 watts.
Nanometer (nm)
A unit of length equal to 10-9 m. Abbreviated nm. Commonly used unit
to define wavelength of light, particularly in the UV and visible ranges of
the electromagnetic spectrum.
Narrow Band Filter
A narrow band filter allows only a limited number of wavelengths to pass
through it. Narrow band filters are usually specified at a specific central
wavelength, a band pass indicating the range of wavelengths that will pass
through it and a % transmission at the limits of the band pass. A 500nm narrow
band pass filter with a band pass of 10 nm with a 5% cut off will pass the
wavelengths between 495nm and 505nm. The transmittance above and below these
wavelengths will be less than 5%.
Neutral Density Filter
A filter that reduces the intensity of the light passing through it without
altering the relative spectral distribution of the energy. Neutral densities
are given by the log base 10 of their attenuation. An attenuation of 100 gives
a Neutral Desnity (ND) of 2. See Optical Density.
Nit (nt) photometric measurement
A unit of measure of brightness (luminance) equal to one candela per square
meter.
Noise Equivalent Irradiance (NEI)
The radiant flux density in W/cm2 required to produce a signal equal to
the inherent noise of the detection system. The input irradiance at which
the signal to noise ratio is 1.
Noise Equivalent Power (NEP)
The radiant power, at a specified wavelength and band pass that will produce
a an output signal from a detector that is equivalent to the inherent noise
in that detector.
Normal
The normal is the axis drawn perpendicular to an illuminated surface. The
normal is the reference from which reflective, diffractive and refractive
angles are measured. A ray with a zero degree angle of incidence arrives perpendicular
to a surface. A ray with a 90° angle of incidence is parallel to a surface
and may not strike it.
Notch Filter
See band eliminator filter.
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Opacity
A measure of a material's ability to block light. It is equivalent to the
reciprocal of the material’s transmittance.
Optical Chopper
A mechanical or electro optical device for passing and interrupting, at
a uniform frequency, a beam of light.
Optical Density (OD)
A measure of the transmittance T through an optical medium. OD =
-log10T. An OD of 1 is equivalent to 10% transmission. A 2 OD
filter would have a transmission of 1%.
Peak Irradiance UV curing
The intense peak of focussed power directly under a lamp. The maximum point
of the irradiance profile. Measured in irradiance units (W/cm²).
Phot (ph) photometric measurement
A measurement unit for illuminance. One phot = 10,000 lux (lx).
Photodiode
A photodiode is a two electrode semiconductor device with an optical radiation
sensitive junction in which the reverse current varies with the illumination.
The wavelength sensitivity is a function of the materials used in the device.
Silicon photodiodes are sensitive through most of the visible spectrum. InGaAs
photodiodes are sensitive in the NIR region of the spectrum. GaP photodiodes
are used for the UV region of the spectrum.
Photodynamic Therapy
The use of optical radiation in the cure of medical maladies. Photodynamic
therapy is used in the treatment of skin conditions such as psoriasis, jaundice
in newborns, and more recently in the treatment of certain types of cancer.
Photometer
A device for measuring luminous intensity or luminance. A photometer employs
a photopic filter which has a band pass that is matched to the human eye response.
The S.I. units employed are lumens and lux.
Photomultiplier Tube (PMT)
A photomultiplier tube is a vacuum device in which a photocathode emits
electrons when exposed to light. The electrons are then accelerated through
electrostatic fields to metal plates where a greater number of secondary electrons
are emitted. This is repeated through several stages. This amplifies the current
by many thousands.
Photopic
Having sensitivity characteristic similar to the human eye response. A
photopic filter will have a band pass between 400 and 700nm with its highest
transmission at 550nm with a spectral responsivity specified by CIE.
Photoresist
A chemical substance which is rendered insoluble upon exposure to light.
By exposing photoresist through a mask, electrical circuits can be created
by washing the unexposed areas and etching the material below. Photoresists
are usually optimized for specific regions of the UV spectrum specified as
UVC, UVB, and UVA depending on the type of lamp employed for the exposure.
Photostability
Many chemical products both medicinal and protective coatings can degrade
when exposed to light. Photostability measurements are made to determine the
principal wavelengths responsible for the degradation and the amount of exposure
(dose) required to create a change that will be deleterious to the product’s
efficacy. In photostability studies the effects from the visible wavelengths
of light must be distinguished from the UV. Typically two detectors are employed
with filtration limiting each to the measurement to only one spectral region.
Pico (p)
Prefix denoting 10-12. One pW = 10-12 Watts.
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Radiance radiometric measurement
Radiant power per unit source area per solid angle. W/m2/steradian.
Radiant Emittance radiometric measurement
Radiant power emitted into a full sphere (4p steradian) by a unit area
of a source , expressed in W/m2. An Integrating sphere is typically used to
make this measurement.
Radiant Exitance radiometric measurement
The radiant flux per unit area emitted from a source.
Radiometer
A device for measuring the intensity or accumulation of radiant energy.
Radiometry
The science of radiation measurement. The detection and measurement of
radiant energy either at specific wavelengths or band passes or as a function
of wavelength over a broad spectrum. The measurement of the interaction of
light with matter as to absorption, transmission, and reflectance.
Ray
The geometric representation of a light path through an optical system.
Reflectance
The ratio of the reflected flux to the incident flux from a surface. In
some cases the measurement may be made with either the specular or diffuse
component of the total reflected flux. Reflectance is expressed as a percent.
Relative Spatial Responsivity
The relative spatial responsivity of a detector indicates the acceptance
angle and percent of incident radiation at that angle that will strike the
detector. The measurement is usually made in comparison to a perfect Lambertian
surface.
Responsivity (spectral sensitivity)
The response or sensitivity of any system as a function of incident wavelength.
In radiometry, it is the output of a device versus wavelength.
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Saturation
A condition wherein the radiant flux density exceeds either or the capacity
of a photodetector to emit electrons in a linear relationship to the incident
flux and/or the current produced by the detector exceeds the capacity of the
electronics to measure the current in a linear fashion.
Scotopic
Relating to the wavelength responsivity of the human eye under dark adapted
conditions.
Sensitivity
The ratio of the output signal from a detector to the input signal. This
may also be expressed as the minimum input irradiance level which will produce
an output signal that exceeds the noise level of the detector; i.e. where
the S/N is greater than 1.
SI
Systeme Internationale d’Unities; the international metric system of units.
Spectral Response
The measure of a detector’s relative sensitivity as a function of incident
wavelength. A typical spectral response curve will display the responsivity
as a percentage at a given wavelength to wavelength of maximum responsivity.
Spectrometer / Spectrograph
A device which measures the interaction between light and materials as
a function of wavelength. A spectrometer is usually a monochromator with an
integrated detector. A spectrograph does not have an exit slit, allowing a
broad band of wavelengths to be measured simultaneously by means of a multi
element detector or photographic plate.
Specular Reflectance
Reflection from a mirror-like surface where in the coherence of the incident
beam is maintained in the reflected beam. This is opposed to diffuse reflection
wherein the reflected light is spread in all directions in a Lambertian-like
manner.
Steradian (sr)
The unit of solid angle subtended by an area on the surface of a sphere
equal the square of the radius of the sphere. One steradian can be visualized
as a conic section with a solid angle of approximately 66°.
Stilb (sb) photometric measurement
A unit of luminance equal to 1candela/cm2.
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Talbot photometric measurement
The SI unit of the quantity of light expressed in lumen-seconds.
Thermocouple Thermopile
A device made up of dissimilar metals in which a small current is produced
as a function of the difference in temperature of the materials at the junction.
Thermocouples can be used for measurement of radiation in the infrared region
of the spectrum.
Transmittance
The ratio of the radiant power transmitted through a material to the incident
radiant power. Transmittance is usually expressed as a percent. A filter with
a 50% transmittance (at a specific wavelength) will absorb half of the light
incident on it and allow half of it to pass through it.
UV (ultraviolet)
The invisible portion of the electromagnetic spectrum with wavelengths
between 1nm and 400nm.
UVA
The portion of the UV spectrum covering the wavelength range between 320nm
and 400nm. This spectral region is used for many medical, UV curing and photolithographic
applications. The earth’s atmosphere (at sea level) absorbs all wavelengths
shorter than UVA. Prolonged exposure to UVA radiation will cause sunburn.
UVB
UVB is the portion of the UV spectrum that covers the wavelength range
between 280nm and 320nm. UVB radiation is typically used in UV curing and
photolithographic applications as well as in certain medical applications.
Exposure to UVB radiation (from lamps or electric arcs) can cause severe sunburn
and cause eye damage.
UVC
UVC is the portion of the UV spectrum that extends from 190nm to 280nm.
UVC is usually employed in water treatment and sterilization applications.
UVC is also used in UV curing and photolithography in microelectronics applications.
Exposure to UVC radiation (from lamps, arcs, or lasers) can cause severe biological
damage.
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VUV (vacuum ultraviolet)
The VUV is the portion of the UV spectrum below 190 nm. Electromagnetic
radiation below 190nm is absorbed by oxygen in air. Physical or chemical interactions
requiring VUV radiation must be performed in a nitrogen purged environment
down to 160nm or in a vacuum chamber below 160nm.
Visible Spectrum (VIS)
The visible portion of the spectrum extends between 400nm and 700nm (per
the CIE). It covers the wavelengths of light that the human eye can perceive.
Watt (W) radiometric measurement
The Watt is a unit of power or work. One Watt corresponds to one Joule/sec.
Wavelength
When electrons vibrate they produce oscillating perpendicular electric
and magnetic fields. The distance between sequential field intensity maxima
is defined as the wavelength. These distances for visible radiation are very
small and are usually expressed in unit lengths of nanometers (nm).
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