LED light sources can be customized to provide the right quality of light for professional tasks.

Currently in microscopes, the existing light source is a quartz-halogen incandescent lamp. LEDs are currently entering the microscope because halogen sources usually dissipate 50W-100W. However, it can be seen that halogen sources are still very advantageous. They are essentially black body radiators.

This means that they produce a continuous spectrum without any protrusions, so any object of any visible color can be seen, and any visible color can be separated by optical filtering.

Clive Beech, component manager of British LED manufacturer Plessey, said, "The advantage of halogen is that it is a good broad-spectrum light source. The spectrum is very uniform and the colors are very good."

each has its own strengths

Optical microscopes are not all. Some microscopes do the same work every day, such as counting white blood cells, which require a narrow range of illumination. Others are universal, among these, the required number of lighting options is "more". The observer may look directly into the eyepiece, or connect to the display of the microscope camera.

Researchers sometimes need excellent and accurate color rendering performance, requiring high contrast between what they are looking for and what they are not interested in, and therefore require the best combination of spectrum and illumination direction.

The first problem of halogen is to protect the sample from heat. Beech said: "It carries a high load of infrared, which is harmful to any tissue samples or organic materials, so you have to filter it out."

LED avoids this layer of filtering, because the standard blue core plus phosphor technology does not produce IR. Plessey optical designer Samir Mezouari said, "Most [LED companies] can simulate blackbody emission spectra. But the challenge is how to get the best performance."

Sufficient optical power is also in place. Beech said: "Our standard white LED is 12V 8W 1000 lm, and the lens is in a 7x7mm package.

The four connection points of the device are flat on a single chip in a square plane, and there is almost no gap between the transmitters, thereby eliminating artifacts.

LED brings change

Although more current is required, large-size chip LEDs are also possible, and the size or number of chip bonding wires needs to be increased. These are potential problems of microscope illumination. Even if Kohler illumination minimizes this effect, the residual image of the light source structure may be superimposed on the view of the object.

Even if the necessary bonding wires are used, the light of the LED source is stronger than that of the halogen lamp. For very sensitive samples, LEDs can perform miniature flash photography to further reduce heat and infrared rays.

Beech said: "You can't remove the halogen to reduce the energy input, but you can use LEDs. You can shoot light with a camera that is always on, and samples that cannot be captured with a halogen bulb can be imaged with LEDs."

It is also easier to use LED for space lighting. For example, Leica allows the LED ring to pass through the objective lens of the reflection microscope, so that light can be generated from only one angle, and the surface features can be displayed.

Due to the long service life of the LED, the LED can be permanently built into the optical instrument, avoiding the rearrangement and calibration steps required when replacing the halogen bulb, and also saves space. There is no need to replace the bulb with finger space and less heat dissipation space. .

contrast enhancement

The first step is to replace halogen with continuous spectrum LED. For contrast enhancement, some microscopes include switchable optical filters, which can be replaced by LEDs of different wavelengths.

Beech said, "You can't always want to use a uniform spectrum for imaging. For example, with blue illumination, the red light sample component will become darker, so the contrast can be increased."

He added, "With LEDs, you can make a broad-spectrum phosphor. Especially for pathology, I will choose the wavelength."

Mezouari said: "Imagine red, green, blue, white LEDs emitting some broad spectrum with specific wavelength peaks. Then, for example, you can turn off red and find the spectrum formula for the specific sample you want."

Camera digital image processing is an important part of the microscope. Mezouari believes that by combining images taken at different wavelengths, highly enhanced images can be obtained. The choice of the wavelength of the LED depends on the work at hand, but if a camera is used, Beech recommends matching the red and green with the blue light and the RGB filtering of the camera chip.

In extreme cases, using only the short-wavelength (blue) light obtained by using the existing LED or filtering out most of the halogen energy can provide higher resolution. There is also some evidence that long-term exposure to strong blue light may adversely affect the human eye.

LED light source

Before science becomes clearer, when the operator may expose himself to strong light, the preventive measure at this time may be to limit the light source to halogen analog or green wavelength or longer wavelength. For severe short-term exposures, there are already photobiological safety standards.

It may be a mistake to completely cancel the spectral illumination, because a sample may only include a single color material that is inconsistent with the light source.

For multi-wavelength LED light sources, the light must be combined in some way. Mezouari said: "You can use a tiny LED array with a small diffuser to get hundreds instead of thousands of lumens, but it's enough."

"If you have enough space, use a separate LED and a mixing rod that is a few centimeters long. Or if you only have red, green and blue, you can use a prism reflector to mix and sometimes add a mixing rod."