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Removing Heat from Optical Systems Using Broadband Hot Mirrors - A Radical New Solution

Heat is an unwanted by product of traditional light sources in many applications. It is of particular concern in bright light microscopy on live samples, where heat can alter normal cell behaviour. In scientific and other instruments the heat from light sources can have a detrimental effect on the detector efficiency and performance. Some detectors are extremely sensitive to extra heat in the system, especially in areas of astronomy where cooled cameras are used. In more everyday life people experience the discomfort of heat concentrated on the area of illumination in operating rooms, TV studios and stage productions.

Figure 1. Left- A cold-reflector lets IR wavelengths (red arrows) pass through the back, while visible (blue arrows) and IR come out the front.

Middle- A hot mirror reflects IR wavelengths back towards the lamp, while visible wavelengths pass through.

Right- A cold-reflector and a hot-mirror combination lets visible wavelengths pass through, while IR wavelengths pass out of the back.

There are two techniques commonly used to reduce the heat at the illumination point. The first, (Figure 1, left) employs a cold reflector behind the lamp that lets infrared heat (IR) out of the back. This method does not eliminate the IR coming out of the front of the lamp, nor does it block re-radiation of the reflector as it heats up over time (explained in more detail below). The second method (Figure 1, middle) uses a hot mirror to reflect the IR back towards the lamp. This method provides more complete IR blocking, but can shorten the lamp life because of heat build-up. Although not widely used, a combination of both methods (Figure 1, right) provides the best performance with regards to lamp longevity and IR blocking. The visible light leaves the front of the lamp while the IR comes out of the back. Typical hot mirrors reflect the near-IR wavelengths, around 900 nm to about 1400 nm.

So the incorporation of a hot mirror in the optical path can be helpful but does not remove ALL the heat. Hot mirrors are filters that moderate heat by transmitting visible wavelengths, while reflecting the infrared wavelengths that would otherwise transfer heat into an optical system. Traditional hot mirrors exhibit three spectral regions - transparency in the visible, reflectance in the near infrared, and an oscillatory pattern in the infrared. Above 1100nm a standard hot mirror will transmit a portion of the light. This will cause excessive heating of the system. Most types of glass will begin absorbing IR at about 2500nm, causing the glass itself to heat up and act like a blackbody by emitting at a much lower colour temperature. So traditional hot mirror designs alleviate but do NOT offer full IR blocking.

Now HORIBA UK Limited can offer a radical new design of hot mirror to address this problem with their new range of unique Broadband Hot Mirrors* which extend the reflecting region across the entire infrared spectrum.

Typical hot mirrors reflect the near-IR wavelengths, around 900 nm to about 1400 nm (Figure 2). The blackbody radiation of a standard incandescent light bulb peaks at about 900 nm with a colour temperature of roughly 3300 K. The curve has a long tail that extends into the IR region of the spectrum (Figure 2). The new design Broadband Hot Mirror* transmits well in the visible region of the spectrum, but transmits almost nothing above 900 nm (Figure 2).

Figure 2. Left- 3300 K blackbody curve (black) with a standard hot mirror (dark blue).

Right- 3300 K blackbody curve (black) with a Broadband Hot Mirror* (dark green). Shaded areas indicate light that passes through the filter.

Note that very little light is transmitted through Broardband Hot Mirror* filter at higher IR wavelengths.

Above about 1400 nm, the standard hot mirror transmits a portion of the light, which can cause heating of glass in the system. Depending on the type of glass used in the system, it may begin absorbing IR at about 2700 nm (Figure 3, left).As the glass absorbs the IR light coming through the hot mirror, it heats up and begins to act like a blackbody itself, emitting at a much lower colour temperature (longer wavelength- Figure 3, middle, right).

The new range of unique Broadband Hot Mirrors* drastically reduces the amount of heat transmitted into a system, including the secondary blackbody effect described above. The Broadband Hot Mirror* reflects the majority of the IR light generated by a 600 K blackbody (Figure 3, left, shaded area, multiplied by 10 for visibility) while the typical hot mirror transmits the majority of the IR light (Figure 3, middle, shaded area).

 

Figure 3. Left- % light absorbed by glass (red trace) with blackbody curve at 3200K (black).

Middle- Light re-emitted from glass at 600K (black) with % reflectance of typical hot mirror (dark blue).

Right- Light re-emitted from glass at 600K (black) with % reflectance of the Broadband hot mirror (dark green).

Shaded areas indicate the light that is absorbed (left) or that passes through (middle, right) the optical element.

The Broadband Hot Mirror* is produced by adding a transparent conducting oxide (TCO) to the back surface of a standard hot mirror, extending the reflecting range far into the IR. This coating transmits well in the visible wavelengths with an average transmission between 80-85%, while acting as an IR reflector with an average reflection > 80%.

Technology

The Broadband Hot Mirror* is produced by adding a transparent conducting oxide (TCO) to the back surface of a hot mirror. The cut-on wavelength of the hot mirror can be adjusted to the application. The TCO absorbs slightly in the visible wavelengths (Figure 1, right) while acting as an IR reflector. It extends the reflecting range of our Broadband Hot Mirror* far into the IR. The top of Figure 4 illustrates the reflectance of a hot mirror with a cut-on wavelength of 690 nm (blue), the reflectance of the TCO (red) and the combined Broadband Hot Mirror* (green). The bottom of Figure 4 is a schematic of the Broadband Hot Mirror* design. The average transmission in the visible is 80% while maintaining a white light appearance.

 

Figure 4. Top- the Broadband Hot Mirror (green) is composed of a Hot mirror (blue) with an infrared reflector (red) on the back side as illustrated in the bottom.

These new designs would be ideal where excessive heat from a light source is creating a problem. Placing the Broadband Hot Mirror* between the light source and sample in a microscope system would enable heat sensitive live cells to be examined without the need for new illumination systems or for complex cooling. Also the new Broadband Hot Mirror* can be used to protect heat sensitive detectors offering lower cost instruments as detector cooling would not be needed. All at significant lower cost!

For a limited period we are pleased to announce that limited numbers of free 25mm diameter samples are currently available to qualified engineers for R&D evaluation. If your system would benefit from a superior Broadband Hot Mirror* that transmits visible wavelengths while extending the reflecting region across the entire infrared, contact ken.norris@horiba.com or call 020 8204 8142.

*The Broadband Hot Mirror filters are design and manufactured by Omega Optical Inc, Vermont, USA. HORIBA UK Limited represents Omega Optical Inc.

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