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Quick View: Electro-Optical Measurement of LEDs

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.(more)

The other type of measurement is Radiometric (Radiometry), which is not limited to the eye response. (more)

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). (more)

Electro-Optical Measurement of LEDs

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:

  1. The total emitted power and/or
  2. 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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