The Influence of Lamp Orientation and Reflector Design On Light Source Performance

Lamp orientation and the reflector design influence the performance characteristics of the light source in a number of ways.

The only way to increase the optical output of a light source is to employ a bigger lamp. However, a higher power lamp will not necessarily result in higher optical output from a light source.

Since light is emitted by the lamp in all directions, the important thing is not the amount of light emitted, but the amount of light collected. The OBB light sources have the highest optical power output as they have the highest collection efficiency.

Optical Power

An ellipsoidal reflector collects and focuses emitted light. Since the ellipsoid encloses a large portion of the solid angle about the arc lamp, over 65% of the lamp’s emission is collected and focused at the output.

This can be compared with the typical performance of a traditional lamp housing design with a spherical reflector and condenser lens where roughly 10% of the light is collected. With identical lamps, the total optical output of the light source is about seven times higher than the traditional housing.

A traditional housing will require a 100W lamp to obtain the same optical output as the light source.

An ellipsoid has two focal points. With the arc lamp accurately positioned at one focal point, the collected light is focused at the second focal point. The monochromator slit or the fiber optic bundle will be positioned at the second focal point in the experiment.

Power Density

For a number of experiments that involves the illumination of a relatively small area, the power density of a light source is a critical specification. Optical power density is a function of the output of the lamp and the size of the area illuminated.

Note that increasing the lamp power does not necessarily increase the power density; in fact, it can actually reduce it. Assuming the optics are the same, the maximum practical power density is a function of arc size.

For instance, a 75W lamp has a luminous area of 0.2X0.5mm, or 0.1sq. mm. A typical 450W lamp has a luminous area of 0.9 by 2.7mm, or 2.43sq. mm. The illuminated area from the 450W lamp is 24 times more than the 75W lamp, but it has only six times more power. The power density of the 75W lamp, then, is four times higher than that of the 450W lamp.


The inside of the arc lamp housing in operation can exceed 150°C. The thermal distortion of the reflector is of concern, since it is the only optical element in the system. Deformation of the ellipsoid shape can drastically reduce the useable output of the illuminator.

The proprietary, thermally-matched optics used OBB guarantee that the shape of the ellipsoidal reflector remains accurate during operation.


Neither a horizontal or vertical arc lamp produces perfectly uniform illumination. The majority of the light is produced near the cathode. When vertically mounted, the illumination between the electrodes is quite uneven from top to bottom.

In the horizontal position, convection currents within the lamp cause slight arc wander. The ellipsoidal reflector produces consistent intensity at the output when the lamp is operated in the horizontal position.

Comparative Field-Test Results

High Efficiency lamp housing yields 20 times better sensitivity and lower operating cost. In an independent comparison of OBB’s Lamp Housing with elliptical reflector vs. a competitor’s condenser (lens/reflector) housing, 100W Hg lamp was mounted in the OBB Lamp Housing, and a 350W Hg lamp was mounted in the competitor’s conventional lamp housing.

Both the lamps were focused through the same monochromator, and then imaged onto a capillary flow cell “sample”. The flow cell emission was then imaged onto a line scan camera, with each pixel representing 1mm of sample length.

As the attached graphs indicate, the smaller OBB lamp housing produced 5.5 times more emission signal with the less expensive 100W lamp than the larger, conventional lamp housing produced with the more expensive 350W lamp.

About Optical Building Blocks

Optical Building Blocks Corporation (OBB) designs, produces and markets state-of-the-art, proprietary electro-optical components and instrumentation used in leading research institutions around the world. Today OBB offers three main product groups of equipment; Interconnectable "Optical Building Blocks", Microscope Accessories, and Bench-Top Instruments.

At Optical Building Blocks we pride ourselves on the technical content of our extensive web page. Our goal is to provide all the information our customers and distributors could possibly need so that they can fully understand the capabilities, specifications and configurations of our components and instruments. And this on-line content is continuously updated with new information, to better achieve that goal.

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This information has been sourced, reviewed and adapted from materials provided by Optical Building Blocks.

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