Detailed explanation of optical microscope light source
The simplest light source used in a microscope is sunlight, which is reflected into the microscope by a mirror. One side of this mirror is flat and the other side is concave. The concave mirror is mostly used for lower magnification. This kind of daylight source is very easy to be utilized. But sunlight is a kind of scattered light, it cannot be imaged on the object plane, and it will cause a lot of flashes on the object, which will reduce the contrast of the image. Of course, using the aperture diaphragm can limit this kind of flash within a certain range when observing at low magnification, and using a flat reflector near the window can often get satisfactory illumination during the clear day. Therefore, daylight illumination is still used in some teaching microscopes and general microscopes for observation.
In modern microscopes, especially in Olympus microscopes, photographic microscopes, and other special microscopes used for various purposes, more artificial light sources are used for lighting. This is because compared with daylight lighting, lighting has uniform light and stable brightness, and all conditions can be effectively controlled. And this light source can image on the object, reduce the scattering, and effectively improve the contrast of the image.
The basic requirements for artificial light sources are: ① have sufficient illumination brightness and sufficient monochromatic light illumination brightness, ② have a sufficiently large luminous surface.
Of course, the requirements for brightness and light-emitting surface are actually not too high. The brightness mainly takes into account the higher magnification, and the larger light-emitting surface is mainly used for low-magnification observation. Excessive brightness can be adjusted through a variable resistor or a medium density filter; the effective area of the light source can often be adjusted with the field of view aperture, and the unevenness of the light source brightness can be adjusted by Kohler illumination or by adding a field glass in front of the light source. Rui to overcome.
In fact, coordination can be achieved between the light-emitting area and brightness of the light source, and these two factors are not isolated from each other. The most commonly used light sources in general microscopes are 40-60W high-voltage incandescent tungsten lamps. These bulbs have a large light-emitting surface and a brightness of several thousand sidings. They are most suitable for use with simpler types of critical illuminators. use. Contrary to what we generally imagine, it seems difficult to understand that a 40W high-voltage bulb should be used instead of a 100W high-voltage bulb when the brightness of the image is insufficient when using a high-power observation. In fact, the advantage of this 100W "strong" light source is only to increase the light emitting surface area. This large surface area is useful for low magnifications, but it does not increase brightness for high magnifications. In addition, high-power high-pressure bulbs emit a considerable amount of heat energy, which is of no benefit to visual observation.
Now often used in microscopes are 12V or 6V low-voltage bulbs. This bulb has a power of 15--m-60W or higher. 2,000-3,000 Xi Ti. This low-voltage lamp has greater illumination brightness than the high-pressure bulb mentioned above, but its light-emitting surface area is only a few square millimeters, which is too small for critical lighting, but this can be used when using Koehler lighting. The condenser lens compensates.
In addition to low-pressure tungsten lamps, there are also high-pressure mercury lamps and high-pressure argon lamps that are often used in modern optical microscopes. The following is a brief description and comparison of the emission spectrum distribution, performance and application of these light sources.
1. Low pressure tungsten lamp
Low-voltage tungsten lamps with adjustable transformers are easy to use and relatively cheap, and can provide satisfactory light output for observation and photography with many microscopes. However, such tungsten lamps have some typical disadvantages, which in some cases are so obvious that other light sources have to be found. The light energy emitted by the low-pressure tungsten lamp has a spectral distribution that is very unfavorable to the microscope. Most of it is in the infrared light or invisible thermal radiation region, and the light emitted in the visible light region below 750nm is mainly of longer wavelengths. Light, in the case of pigeon lamps using ultra-high voltage, there will be some increase in light output in the visible light range, but this will correspondingly reduce the life of the bulb, and the increase in light output is also unstable.
Another problem involved with tungsten lamps is that the bulb gradually dims with use, as tungsten evaporated from the hot filament deposits on the inner surface of the bulb, resulting in a gradual decrease in light yield and emitted light spectrum. Changes in distribution. The tungsten-halogen lamp that has appeared in recent years can be considered as an effective improvement to the low-pressure tungsten lamp. This lamp is filled with a halogen gas (such as iodine) temporarily combined with tungsten in the glass bulb, from the heated filament to The gaseous form is emitted, and the confined tungsten is redeposited on the filament, the halogen gas is released and the cycle repeats. Since this lamp has the highest light yield of all tungsten lamps used in microscopes and a lamp life of thousands of hours, it has become very popular in microscopy, especially in microscopy. But because the filaments of this kind of lamps are small and dense, the temperature of the filaments is very high, which can reach 3,000^-3,1001, so they emit a large amount of heat. The thermal filter absorbs some of the heat.
2. Off-pressure mercury lamp
This is a gas discharge lamp made of quartz that emits mercury between two high-voltage electrodes inside the discharge vessel. It has a more dispersed banded spectrum in the visible range, as opposed to the continuous spectrum of a tungsten lamp. In a comparison The low continuous base has a narrow and high emission band at a certain wavelength. Because it has special emission peaks at 546, 436 and 365nm wavelengths, when selecting through the selection filter, it is suitable for fluorescence microscopy Said to be a very effective light source. Due to the limitation of the banded spectrum, good contrast cannot be obtained on stained sections, however, it is still a good light source with a considerable light energy emission in the optimal part of the spectrum.
3. High voltage failure lamp
This is a relatively new type of gas discharge lamp that emits nitrogen gas, and it has more advantages. It has a continuous emission spectrum in the visible light range, and has a certain emission continuous spectrum in the ultraviolet light part. It is considered to be the most effective general-purpose light source today. At the same time, this high-pressure lamp can provide extremely high brightness stably, so it is a state-of-the-art light source and has an irreplaceable position in some special microscopes.
