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There are several areas of “light” measurement
that are used to define the performance parameters
of an LED. For visible light LEDs,
what is of primary interest are Photometric (Photometry)
measurements which relate to measurements of visible
radiation as the “standard” human eye responds
to it. As such, Photopic (cones are functional),
Mesoptic (rods and cones are both functional),
and Scotopic (only rods are functional – i.e.night
vision), are the key psychophysical elements.
The other type of measurement
is Radiometric (Radiometry), which is not limited
to the eye response. Radiometric measurements
are used to cover the full spectrum of radiation/photons
emitted from the LED (visible, plus ultraviolet
and infrared).
Photometric units of measurement are important
for LED product designers focusing on visible
LED product applications (such as Big Screen Video
Displays), while radiometric measurements are
important both to packaging specialists as well
as for end-product designers focused on applications
requirng understanding as to a devices ’emitted
radiation profile (such as discrete LED materials
component engineers or photo-dynamic-therapy -
PDT apparatus providers). Consequently both types
of informational data are equally important to
the industry at large.
UNIROYAL Optoelectronics, in defining its products
to industry has decided to provide both Photometric
and Radiometric units of measurement whenever
it can. These measurements are conducted with
the industry standard drive condition of 20 mA
and are provided in our Product Data Sheets. Additional
data can be provided by UOE upon request if necessary.
It is important to note that the
information measured and listed by UOE is data
obtained solely based on bare die measurements
and does not include any encapsulating index matching
material(s) applied to the LED surface(s). Data
is obtained via test probe head or with an integrating
sphere of bare die only. These bare die measurements,
provide the LED die user the basic information
with which to devise their corresponding packaging
design approach which, if properly implemented
will achieve higher light output through their
selected packaging approach and corresponding
drive level (i.e. the Transfer Function).
Photometric Measurements: The “standard”
human eye or “standard observer” (photopic) curve(s)
has been established by the Commission Internationale
de l’ Eclairage (CIE). The most widely used version
is the CIE 1931 Chromaticity Diagram. The eye
response is not a constant over the entire visible
range. It peaks at Green 555 nm and falls off
sharply toward the Blue at 400 nm, and in the
Red at 700 nm. Detectors required for taking this
type of measurement must be spectrally corrected
so that their response matches the CIE curve selected.
This correction is usually made by adding a specially
designed photometric filter to the detector. Although
many common terms are used to define photometric
light output, the basic unit of measurement of
light is the lumen. All photometric quantities
involve the lumen. The photometric quantities
of particular interest associated with LED measurements
are: Luminous Flux: The total luminous power emitted
from a source (lumens), and Luminous Intensity:
The luminous flux emitted from a source per unit
solid angle in a specified direction (lumens per
steradians or candelas).
Radiometric Measurements: The basic radiometric
unit of optical flux (power) is the watt and all
the various derivatives in this system of measurement
will contain the watt in their terminology. These
terms are more scientific in their usage in that
one watt of radiation in the ultraviolet is equal
to one watt in the visible and one watt in the
infrared range as well. Radiometric measurements
are determined accurately when both the spectral
power distribution of the light source measured
is known along with the absolute responsivity
of the detector are known. The absolute spectral
responsitivity of the detector is determined through
a transfer from a known National Institute of
Standards & Technology (NIST) calibrated detector.
These two functions, the lamp specific power density
(SPD) and the detector absolute responsitivity,
are then combined and resulting light source specific
scaling factor applied to the photocurrent read
from the detector. The radiometric quantities
of particular interest with LED measurements are:
Radiant Flux or Power: The time rate of flow of
radiant energy (watt), and Radiant Intensity:
The radiant flux emitted from a source per unit
solid angle in a specified direction (watts/steradian)
LEDs are characterized by either of these two
systems in two different ways:
- The total emitted power and/or
- the Intensity.
Emitted power is all the flux (power) generated
by the LED (lumens or watts) and is collected
and measured with no regard to the direction of
the flux. Intensity is useful since it is the
flux per unit of solid angle directed toward the
observer, and is most useful in a packaged LED
lamp as the light in this lensed format is directed
to the observer, usually along the axis of the
lensing or optical system. Luminious intensity
is expressed in terms of lumens/steradian which
is the candela. Radiant Intensity is in terms
of watts/steradian.
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