Lednium Series Optimal X
OVTL09LG3x Series
Power input is 1 watt; however, some power is converted into light energy. Assuming this is of the order of
200mw, the adjusted value of δT is 2.29K. The calculation now assumes that all of the dissipation, 800mw of heat,
is conducted along the thermal path, thereby ignoring any conduction and subsequent radiation that is not direction-
ally normal to the surfaces considered, ie: conduction through the encapsulant material vertically away from the
board, and conduction horizontally away from the heat source. The calculation also assumes that there is no contri-
bution to thermal resistance at the boundaries between material layers. In practice it is improbable that perfect
transfer will occur at these transition regions, even though the bonding between layers in this example are of high
quality. In general, the calculation indicates that the measurements below are of the order of magnitude that can be
expected.
The alternate matrix product range is of a much more complicated thermal design, which does not lend it-
self to a simple theoretical calculation similar to that shown above. There are multiple incident heat sources, parallel
heat conduction paths, and significantly larger surface area for stray radiation, eg. Cup above has a surface area
available for stray radiation of approximately, 25mm2 per watt of input power. A 10-watt matrix product has approxi-
mately 92.5mm2 of exposed surface per input watt.
Measurements
The key to an accurate measurement of thermal resistance is to obtain a reliable value for the junction tem-
perature (Tj). Since the die itself is, and must be, encapsulated during testing, and the junction is contained within
the structure of the die, direct measurement of the junction temperature by normal means is not possible.
Two methods of non-contact thermography are available, both of which rely on emitted infrared detection.
Infrared imagery by calibrated radiograph is a possibility; however, in the instance of a cup product only a
small value of δT is expected which makes accurate estimation of the actual temperature gradient difficult using col-
orimetry.
The alternative measurement type is digital infrared thermography. This means there is an inherent uncer-
tainty in the calculation algorithm, which sometimes gives results considered unacceptably inaccurate. In this in-
stance absolute accuracy is of secondary importance because the value to be determined is a temperature differ-
ence (δT) which requires only relative values – any error in a first reading will also be present in subsequent read-
ings that are about the same value. The difference between readings is accurate.
The other significant drawback to infrared thermometers is a limitation to minimizing the spot size over
which the measurement is made. This poses a difficulty for small assemblies like an LED cup, and in particular the
added complication that the calculated temperature is an average value for the area being interrogated further com-
plicates the issue. Another concern is sometimes raised about the ability of this type of instrument to detect a
heated surface beyond the closest transparent radiating surface. This is a significant issue for far field measure-
ments; however, it is simple to demonstrate that this does not hold true for the near field, and particularly when the
incident beam has a known focal length.
OPTEK reserves the right to make changes at any time in order to improve design and to supply the best product possible.
Issue B.2 07/08
Page 13 of 14
OPTEK Technology Inc. — 1645 Wallace Drive, Carrollton, Texas 75006
Phone: (972) 323-2200 or (800) 341-4747 FAX: (972) 323-2396 visibleLED@optekinc.com www.optekinc.com
