Pro Lite Technology

LED & OLED Applications

LEDs

LEDs are being used in ever increasing numbers in lighting and display applications, ranging from solid state lighting luminaires, indicator lights, display backlights, large area video displays and signage. The human eye is very sensitive to brightness and colour differences and the problem with LEDs is they cannot be manufactured with consistent optical properties, even within the same batch. This makes measuring LED output an important activity, but their small size, narrow spectral emissions and directionality mean that traditional photometric and colorimetric measurement methods are not always the most appropriate. Here at Pro-Lite, we have been working with LEDs for over 20 years and we understand the science of optical radiation measurements. Choosing equipment from us means you can tap into our expertise and have confidence in your results.

Luminous Flux Measurements

Luminous flux is the total photometric power emitted in all directions from a light source, measured in lumens (lm). With the proliferation of high brightness LEDs used for illumination, measuring the total flux of LEDs has become widespread. With OLEDs, measuring luminous flux allows you to compute the photoluminescent quantum yield of the material. Luminous flux can be measured using a Labsphere integrating sphere photometer, the diameter of sphere being dictated by the size of the device under test. For improved accuracy, a Labsphere integrating sphere spectroradiometer measures the spectral radiant flux and computes the luminous flux and colour parameters. Uniquely, spectroradiometers also provide colour rendering information and can apply both daylight (photopic) and night time (scotopic) observer functions.

An integrating sphere can be used to measure LED flux in two ways. Larger integrating spheres hinge open to allow the sample to be placed in the centre of the sphere. In this configuration, you record the luminous flux from both single emitters as well as LED arrays integrated over 4pi steradians of solid angle. Alternatively, you can place the LED at the input port on a small sphere or the external source port on the wall of a large sphere and record the partial (or forward) flux integrated over 2pi steradians. This latter approach is well suited to LED downlighters and has the benefit that the LED mount, heat sink and drive electronics remain outside of the sphere, which minimises absorption errors.

A recent development is the Labsphere "Half Moon" integrating hemisphere. These are ideally suited to measuring the output of LED-based solid state lighting. By mounting the sample and its drive electronics outside of the (hemi)sphere, thermal management problems are much reduced and measurement speed and accuracy are improved.

Integrating spheres are the fast way of measuring flux. However, a goniophotometer can also yield a total flux value as the intregral of 2pi or 4pi steradian directional luminous intensity.

Luminous Intensity Measurement

Luminous intensity is the luminous flux emitted per unit solid angle, measured in candelas (cd). Intensity is what you measure when the lamp is a point emitter, in other words when you are in the photometric “far-field”. Conversely, when you move up close to the light source (the “near-field”), you transition to measuring luminance in candelas per sq. meter (cd/m2). Consider an array of 100 by 100 individual LEDs used in a video display panel. You would measure the luminous intensity of each LED but when you view the display as a whole, you would measure the luminance (or brightness). However, if you were to move sufficiently far away from the LED panel that it becomes a point source, you would then measure the luminous intensity of the panel.

Intensity should be measured in a defined direction; unless the lamp is isotropic, intensity will vary with direction of view. Intensity measurements can be performed with an illuminance photometer (or for improved accuracy a spectrometer configured for spectral irradiance measurements) placed approximately 10 to 20 times the source size away. This records the illuminance (in lux), and the conversion from lux to candelas is performed by multiplying the illuminance by the square of the distance between the lamp and the detector.

The problem with LEDs is that they don’t conform to the ideal of a point source which meant in the past that measurements of far-field intensity were highly instrument-specific. CIE's publication number 127 addresses this by defining standard measurement geometries. The CIE geometry defines – in effect – a near-field, or average intensity. A photometer (or spectroradiometer) with a collection area of 100mm2 views the LED at a distance of either 100mm (condition B) or 316mm (condition A), equivalent to view angles of 0.01 or 0.001 steradians respectively. CIE 127 applies to the measurement of individual LED emitters only. The Labsphere I-1000 and I-2000 are sensor heads that provide for measurements of the spectral radiant intensity of individual LEDs in accordance with CIE 127, condition A & B.

Illuminance Measurements

Illuminance is the luminous flux per unit area received at a surface, measured in units of lumens per square meter or lux (lx). For a point light source in the photometric far-field, illuminance decreases
with the square of the distance away from the lamp. This is the so-called “inverse squared law”. The illuminance at a surface tilted at an angle θ to the direction of illumination is reduced by the cosine of θ. It is for this reason that an illuminance photometer (or for improved accuracy a spectrometer configured for spectral irradiance measurements) is normally equipped with a cosine diffuser which scales the off-axis illuminance to take account of the reduced illuminance at higher angles of illumination. Luminous flux and intensity are intrinsic properties of a light source, whereas illuminance varies with the distance from the source. Consequently, measurements of the illuminance of a luminaire must be performed at a defined distance.

An alternative approach can be taken when measuring the spatial illuminance from sources such as projectors or lensed LEDs. An imaging photometer is a powerful, CCD-based spatial light and colour measurement instruments that provide for increased productivity compared with traditional photometers and colorimeters. The beam of light is shone onto a matte white screen and the imaging photometer measures the illuminance, luminous intensity and colour of literally millions of points simultaneously. The measurement area can be selected in software after the measurement has been made, and moreover, any number of analysis points can be defined - and recalled - as required. In addition, because the camera views the whole illumination pattern at once, localised illuminance and colour differences can be easily detected – artefacts that spot measurements might miss.

Luminance Measurements

Luminance is the luminous flux per unit area per unit solid angle emitted from a light source, measured in units of lumens per square meter per steradian, or in candelas per sq. meter (cd/m2). Historically, luminance has also been measured in units of nits, where 1 nit equals 1 candela per sq. meter. Luminance is the photometric term for what we think of as "brightness". Luminance is what you measure when you are in the near-field viewing an extended light source; intensity is the related far-field measurement for a point source. Luminance photometers (or for higher accuracy spectroradiometers) employ a lens to image a defined area or "spot" on the light source. Unless the light source is Lambertian, luminance can vary with viewing angle, hence it is necessary to specify the direction of view when making luminance measurements. Luminance is the property you would measure with an OLED panel or an array of LEDs, for example an LED video display.

The Konica Minolta LS-100 and LS-110 are simple, affordable, hand-held photometers that provide for spot luminance (brightness) measurements. The Konica Minolta CS-100A is the colorimeter version of the LS-100. The accuracy of filter photometers is always reduced when measuring narrow spectrum light sources such as LCDs and LEDs. The Konica Minolta CS-200 is a hybrid "spectral colorimeter" that combines the ease-of-use and relative affordability of the CS-100A with a colorimetric accuracy approaching that of the research-grade Konica Minolta CS-2000 spectroradiometer.

As the name suggests, "spot” photometers measure the brightness and colour of an LED or display one spot at a time. At most, you can select from up to five measurement spot sizes, either by fitting close-up lenses or by selecting alternate measurement apertures in the photometer. If a more flexible and productive (albeit more costly) solution is more your thing, you should consider an an imaging photometer.

Imaging photometers are powerful, CCD-based spatial light and colour measurement instruments that provide for increased productivity compared with traditional spot photometers and colorimeters. Whereas a spot photometer can only measure the brightness and colour of one point on a display or light source at a time, a CCD-based imaging hotometer can measure literally millions of points simultaneously. The measurement spot size can be selected in software after the measurement has been made, and moreover, any number of analysis points can be defined - and recalled - as required. In addition, because the camera views the whole display at once, localised luminance and colour differences (defects) can be easily detected automatically – artefacts that spot meters might miss.

Chromaticity, Colour Temperature & Dominant Wavelength

The colour of an LED is expressed in a variety of ways. The perceived colour of a light source depends upon its spectral power distribution, the human eye's tristimulus response and the relative amounts of red, green and blue in the light. To simplify the reporting of colour, we normally quote the CIE (x,y) chromaticity coordinates (1931 2° observer).

However, a further simplification can be made and for white light LEDs, we can refer to the correlated colour temperature (CCT), reported in units of Kelvin (K). An LED with a CCT of about 3,500 K is referred to as being "warm white" due its output being weighted more to the red end of the spectrum. Conversely, an LED with a CCT of about 5,000 K is considered to be "cool white" due to the light being weighted more to the blue end of the spectrum. Ironically, the higher the (colour) temperature, the "cooler" the light.

For coloured LEDs, CCT has no meaning and instead we use the simplified colour metric called dominant wavelength, measured in nanometers (nm). It is a measure of the hue (or colour sensation) produced by the LED and should not be confused with peak wavelength.

Colour is measured either using a tristimulus filter colorimeter, a spectroradiometer or a spectrometer configured for spectral irradiance measurements. The accuracy of filter colorimeters is always reduced when measuring narrow spectrum light sources such as LEDs. For improved accuracy, a spectrometer or spectroradiometer measures the spectral power distribution and computes the photometric and colorimetric parameters. Uniquely, spectrometers and spectroradiometers also provide colour rendering information.

Colour Rendering Measurements

Colour rendering in an important metric for solid state lighting luminaires. The problem of poor colour rendering can be easily understood if you consider that you can mix the light from a blue and a red LED to create white light. However, if you then illuminate a green surface with this light, the green surface will not appear green. The colour rendering index (CRI) defines how well colours are rendered by different white light LEDs compared to a defined standard illuminant. Colour rendering can only be computed for a given light source if you know the full spectral power distribution, hence CRI cannot be measured using a tristimulus (filter) colorimeter. Instead, CRI must be measured using a spectroradiometer or a spectrometer configured for spectral irradiance measurements.

View Angle Measurements & Standard "Photometric" Data

The instrument used for measuring the angular variation from a lamp or luminaire is called a goniophotometer. The goniophotometer measures luminous intensity versus angle in the far-field. Click here for details on the goniophotometer systems which Pro-Lite supplies
Related Products
Presentation

> Testing Solid State Lighting

Pro-Lite was invited to give a talk on the metrics and methods for testing solid state lighting at the Ultra Efficient Lighting seminar held during the Photonex exhibition in October 2009.

Brochures
Case Studies
Knowledge Base
Have a Question?

Why not use our convenient enquiry form?

> Click Here

Buy Online

This product can be ordered via the Pro-Lite on-line shop.

> Click Here

 Need a
 Measurement?

Pro-Lite offers measurement services for LEDs and solid state  lighting luminaires.

> Click Here