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Light source
Authors
Abstract
The description of the cold light source covers all aspects of this piece of equipment.
The technical key steps of the chapter are presented in a step by step way: principles of function, ideal requirements for laparoscopy, available material, advantages/disadvantages, usage and adjustments, usage problems, criteria of choice.
The technical key steps of the chapter are presented in a step by step way: principles of function, ideal requirements for laparoscopy, available material, advantages/disadvantages, usage and adjustments, usage problems, criteria of choice.
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 Publication
2003-04
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E-publication
WeBSurg.com, Apr 2003;3(04).
URL: http://www.websurg.com/doi-ot02en306a.htm
URL: http://www.websurg.com/doi-ot02en306a.htm
Light source
1. Introduction
An excellent light source is mandatory for safe laparoscopic surgery. High-quality light sources such as those used in laparoscopy require fastidious maintenance of both the bulbs used in the light sources and the fiber-optic cables.2. Principles of function
• Spectrum of light
• 1
Luminescence can be produced by passing an electrical current through a tungsten filament, thereby heating it; an example of this is an incandescent lamp. The filament can be placed in a vacuum as in a standard electric light, or it can be bathed in gas, as in a halogen lamp. Halogen lamps are becoming widespread, and are used in automobiles and in the household. They are equally present in the medical field. Sources of light emitting between 300 and 500 W have been used in endoscopy for the last 20 years. Unfortunately, the spectrum of light delivered from these light sources is not well adapted for surgical use. The spectral temperature of color of these lights (3200 degrees Kelvin) is too low and too different from white sunlight (5600 degrees Kelvin on average).• 2
A more suitable light source for laparoscopic cameras involves the creation of an electrical arc in a metal halide atmosphere (Hydrargyrum Medium arc-length Iodide or HMI) or in xenon. This electrical arc, much in the same way as the flash used in photography, without a filament, emits a light that is much closer to actual white sunlight. The HMI light sources emit a spectral temperature of color of 5600 degrees Kelvin and the xenon sources emit a spectral temperature of color of approximately 6000 degrees Kelvin on average for a power of approximately 300 W. These lamps generate 4 times the candle power of the filamented halogen lamps. Unlike these, however, arc-generated lamps have a spectral temperature that gradually decreases with use. For this reason, a white balance is required before each usage of this type of lamp, and the bulbs need replacing after 250 to 500 hours of usage, depending on the type of lamp. Only the last 3 types of lamps discussed (arc light, HMI or xenon) are recommended for use in laparoscopic surgery.
• Arc lamps/characteristics
HMI lamps and xenon lamps have the following characteristics.• Light source cables
The light source cable is composed of a bundle of optical fibers buffed at each end. There is an adapter on each end of the cable for the light source and for the laparoscope. There are 3 systems of fixation, which permit the use of screwed adapters, so any optical source can be connected with any type of laparoscope.Options
Certain companies have proposed using cables that transfer light via a ‘gel’ or liquid crystal. These systems are very fragile and become hot, and they are currently not in use.
• Fiber-optic cables
• Optical fiber
Fiber-optic cables are constructed from an assemblage of fibers of either glass or plastic with a diameter of 20 to 150 microns each. These fibers are able to transmit light without generating heat.3. Ideal requirements/laparoscopy
Light sourcesAll companies manufacturing laparoscopic cameras also produce light sources. The manufacturers recommend a power of between 100 to 450 W. Regular use of a high power light permits a diminished camera aperture and a better depth of field. For these reasons, we recommend using light sources with a power greater than 250 W.
It is better to use equipment that displays the usage hours to keep track of the light source status.
Light source cable:
A good quality cable is absolutely necessary to transfer light from the light source to the laparoscope. The adapters also have to be fixed snugly so there is no motion. In the absence of good connections between the cable and a light source, there is a loss of light and a generation of heat.
4. Available material
• Light sources
The spectrum of possibilities ranges from a halogen light source at 100 W to a xenon light source at 400 W. The light sources in the cameras are designed to work together, therefore it can be difficult to disassociate the two. Some light sources contain 2 types of arc lights (halogen and xenon). It is therefore possible to choose between halogen and xenon, depending on the settings of the camera.• Light source cable
Cable adapter sizes:- for a 10 mm endoscope, a diameter of 5 to 6 mm is recommended for the adapter for the light source cable;
- for a 5 mm endoscope, a diameter of 2 to 3 mm is recommended;
- for a 2 mm endoscope, a diameter of 1 to 2 mm is recommended.
The larger the diameter of the cable, the more it heats the endoscope and thereby reduces its longevity.
Cable length:
A cable length of 1.5 to 3 m is generally recommended for a light source.
5. Advantages/disadvantages
All types of laparoscopic cameras can be adapted to all types of light sources. Certain combinations of camera and light source have systems for automatic control of light intensity delivered via an electronic ‘diaphragm’ in the camera. If the light source and the camera have not been properly adapted to each other, this automatic functioning may not work properly.
The connections for these devices pose no particular problems, as most systems are compatible with each other. However, connections for certain light sources may sometimes require an adapter.
6. Usage and adjustments
• Usage
While a cold light source is easy to use, thorough knowledge of its function will optimize usage.Over a period of time, wear on the arc lamps is indicated by a decrease in the color temperature emitted. This gradual modification in the color temperature accounts for the need to adjust the white balance each time the endoscope and cold light source are used together. This allows the endoscope to adapt its neutral white position to the color temperature emitted by the cold light source. Practically speaking, this is apparent with new lamps and before setting the white balance, by a bluish appearance of whites that is due to a very harsh light when a new bulb is used. As wear on the bulb increases, this initial color gradually shifts toward a pinkish and reddish cast. This indicates the distortion of the color emitted by the lamp and requires an adaptation of the camera’s sensitivity to this light. Ideally, the white balance should be adjusted after about 5 minutes, when the light source has reached its operating temperature.
Bulb life:
The color temperature emitted by an arc lamp should never be less than 3500 degrees Kelvin. As the color temperature decreases progressively, the life of these bulbs, for surgical use, must never exceed 250 hours for a metal halide bulb or 500 hours for a xenon bulb.
Connection of light source cables:
The operating temperature of these lights is several hundred degrees, but filters decrease the heat transmitted by the cables. It is important, however, to ensure that the light source cable is properly plugged into the light source. A faulty connection can cause major heating of the light source unit/cable junction and lead to destruction by burning of the light source cable.
• Adjustments
• Light source cable
Cold light sources that deliver medium power (200 to 300 W) are normally used at maximum power. In this case, the role of the camera’s diaphragm is to adjust the quantity of light transmitted. When the intensity of the light source is higher, and in ideal conditions, full power is not transmitted, in order to avoid overexposure of the operative field.Light source cable:
Use of a cold light source cable poses very few problems. It is mandatory, however, to ensure perfect connections between the light source and the cable, and between the cable and the endoscope. The life of a cable, although it is theoretically unlimited, is limited in practice by the gradual rupture of the fibers due to repeated micro-shocks. A light source cable should be changed each time a light source bulb is replaced.
Diameter of plugs:
Depending on the manufacturers and models, various joining systems exist for the endoscope and plugs. Three connecting systems for the endoscope are shown here. Most endoscopes are supplied with adapters that can be connected to the 3 types of cable plugs.
• Connection for light source
Many light source systems use jawed systems into which any kind of light cable can be inserted. Other manufacturers propose specifically devoted connection systems. For the latter case, adapters exist, permitting transfer from one connection system to another. The fit of these adapters must be exact in order to avoid an overheating of the connection that may occur if there is an imperfect contact between surfaces.7. Usage problems
• Light sources
Various problems can alter the luminosity of the operating field. In all cases, the light source and cable should be checked, as well as all connections between the light source, endoscopic camera and monitor.An old bulb can cause numerous alterations to the image quality, and must be changed.
When blood is present in the operative field, most of the light is absorbed by the hemoglobin and is therefore not reflected. The field darkens rapidly if the lamp’s light reserve is insufficient. This is easily noticeable because the blood in the operative field appears to be black. With a new, good quality bulb, the blood appears red.
Blurry image: due to an insufficient light source, the opening of the camera’s diaphragm is at its maximum, reducing the depth of field and the sharpness of the image.
Close working distance: when the light source is insufficient, the surgeon tends to move the laparoscope closer and closer to the operating field in order to keep it sufficiently well lit. This interferes with operative maneuvers and makes the case more difficult.
• Light source cable
• Complications 1
The main problem associated with fiber optic cables is their fragility and susceptibility to damage with excessive manipulation. The main causes for rupture of the cables are:
- direct impact;
- major twisting when rolled too tightly;
- crushing (usually by a cart that is accidentally rolled over the cable).
• Complications 2
Another complication that affects light source cables is a burning of the fibers at the tip of the cable due to connections without direct contact between the light source and the cable, or between the cable and the endoscope. This damage also produces subsequent diminution of light transmission.• Preserving light source cables
The best way to preserve the longevity of a fiber optic cable is by avoiding impact and by careful handling. Their curvature radius must be respected: significant rupture of the optical fibers is observed when the cable is wound in a circle whose radius is smaller than 15 cm.8. Criteria of choice
Light sources:A 3 CCD camera requires a more powerful light source than a mono-CCD camera.
For a mono CCD camera, the minimum light source (arc lamp, metal halide or xenon) should be 200 to 250 W. For a tri-CCD camera, the minimum light source (arc lamp, metal halide or xenon) should be 250 to 350 W.
It is mandatory to replace the bulb after the recommended period of time (250 hours for a metal halide lamp, 500 hours for a xenon lamp).
A chronometer that is built into the light source is preferable to an indirect system.
Connections:
When purchasing a unit, it is important to make sure that the connections for the endoscope, light cables and light source are compatible. Even minor differences can make it impossible to connect one system to another.
9. Conclusions
New high-resolution endoscopic cameras provide excellent visualization of the operative field, but require powerful light sources for efficient lighting. In order to obtain a high quality image, this part of the unit, which is inseparable from its light source cable, must not be neglected.