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Xenon high-pressure arc lamp (CIE)

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Metal Halide lamp "YOKO lamp"

Figure 5. Spectral power distribution of various lamps permitted for use by ICH guideline (courtesy of Dr. M. Matsuo, Tanabe Seiyaku Co., Japan).

It is difficult to control the amount of mercury added during production. Only a very small drop is added but it is difficult to control this amount. For this reason, in particular, the intensity of these lamps will generally show a large variance. This is especially true of the "hardware store", mass produced, home use, monohalophosphate lamps. The multiphosphor lamps will generally show less of these problems but of course cost 5-20 times as much.

Additionally, fluorescent tubes are coated by first filling them with a slurry of the phosphor(s), then letting them drain before putting the tubes on a slightly inclined roller bed to allow them to dry without too much settling. These tubes are then put into an oven and fired to convert the slurry into a fine coat on the inside of the tubes. One outcome of this process, used particularly for the low price commercial market, e.g., "cool white" tubes, is shown in Figure 6. The differences in intensity noted, not readily evident in this black and white picture are actually color differences along the length of an individual tube.

This color difference, ranging from white to yellow to dark blue, results from difference in coating thickness of the lamp from one end to the other and the intensity of the fluorescence activating UV radiation. The thinner the coating the darker the color. The thicker the coating the whiter the color. Triphosphor and other more thickly coated lamps will not have this same problem. This is a problem specific to the thinly coated, mass produced and therefore cheap "cool white" lamps.

One advantage of fluorescent lamps is the very low amount of infra-red radiation. This advantage is not always found in practice. Since these lamps are low in intensity, it is not uncommon for several to be used at the same time. This practice leads to heat buildups that require cooling fans or cabinets.30'31

One should also be cognizant of the fact that not all fluorescent "daylight" lamps are equivalent. This problem is well illustrated in Figure 7. Here I have plotted the Relative Percent Energy of the CIE D65 standard, the ICH "cool white" lamp, two US and one European "daylight" lamp versus wavelength. The Relative Energy (%) was obtained by dividing the individual measured energy at one sample point by the total amount of energy over the 300-700 nm range and multiplying this value by 100. This procedure normalizes the data. If all of the lamps were equivalent, all of the curves would be superimposed.

The data obtained were plotted only up to the 580 nm data point to emphasize the differences in the more active and relevant 500 nm and below region of the spectrum. It is readily apparent that fluorescent "daylight" lamps are not good replicators of the D65 standard.

9.3.2 Xenon lamps. The power of Xenon, high-discharge lamps52"54 to provide very intense white radiation led to their development as a radiation source in the 1930 - 1940s. The ability of this source to reproduce solar like spectra at high illuminance levels was recognized from the outset. Development was rapid due to their use in the war effort as lamps for search lights. Over the years the major problems that thwarted their adoption in testing laboratories, ozone formation and bulb explosion have been eliminated. Ozone formation was eliminated by the use modern UV absorbing glasses and the explosion problem (for high pressure short-arc lamps) greatly reduced with the development of stronger glasses. LAX (long-arc Xenon lamps) lamps are medium to low pressure lamps.

Figure 6. Color variation along the length of single halophosphor "cool white "fluorescent lamps (personal communication).

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Figure 7. Spectral power distributions of different fluorescent lamps. US #1, US #2 and European lamps advertised as daylight lamps.

Wavelength (nm)

Figure 7. Spectral power distributions of different fluorescent lamps. US #1, US #2 and European lamps advertised as daylight lamps.

Xenon lamps consist of two electrodes enclosed in a quartz bulb or tube, which contain either mercury and Xenon or Xenon only as a fill gas. When ionized, SPDs of xenon + mercury or xenon only, are produced. Short-arc xenon, xenon + mercury lamps are used primarily for Sun Protection Factor determinations. The mercury is added when a more intense source is desired. However, the addition of mercury does add the additional mercury spectral lines and give data that do not correlate as well with natural sunlight.

The short-arc lamps have a smaller area of illumination and are more subject to intensity variations due to more significant changing arc lengths with time. Additionally these lamps emit line spectra from volatilized electrode metals and their impurities. The volatized metals also deposit on the cooler glass surface, resulting in a darkening of the glass inner surface and a reduction in the emission intensity.

Long-arc xenon lamps (LAX) (5 mm or more in arc length) have been used for several years and are the most common source used with the appropriate filters to artificially produce the CIE D65 SPD. They are, as noted, simple in construction, available world-wide, well characterized and accepted by practically all photostability testing groups such as the ASTM, ISO, DIN, JIS, etc.

These lamps are available in several wattages ranging from 150 to 6500 watts. Lamps larger than about 1500 watts generally require water cooling to reduce the large amount of infra-red energy emitted. The water cooled lamps emit a reduced amount of infra-red radiation. It is common to use these higher wattage lamps under conditions where the environment is controlled,55 therefore the heat generated is not a significant problem.

Other methods of reducing this xenon lamp heat burden have been applied to currently available photostability instruments such as the use of cooled air and dichroic filters.

LAX lamps have a large area of illumination, emit a pure xenon gas spectra, change intensity less with age and show little darkening or metal deposition with age. Xenon lamps are also the basic reference lamps used for international standards of daylight, D65, or ID65 (window-glass filtered daylight).

With the concurrent development of the long-arc Xenon (LAX) low to medium pressure lamp the usage of this lamp was extended to other areas such as studio lighting and areas of photostability testing requiring large areas of illumination. The use of these lamps at higher wattages, i.e., greater than 1500 watts, required water cooling of the quartz envelope. This additional cooling did much to eliminate the infra-red radiation present in all xenon lamps.

Currently available lab top photostability testing equipment uses air-cooled LAX lamps of a wattage limit of 1500. Higher power lamps such as that used by Forbes and associates55 require water cooling but have the advantage of reduced infra-red radiation at the sample.

9.3.3 Metal halide lamps. Metal halide lamps, first discovered at the end of the 18th century, did not come into practical use until the early 1960 s.56-59 These lamps have been widely used to light large industrial areas, stages, studios and as street lights, i.e., sodium and mercury vapor lamps. The metal used, usually comes from the Group Ill/Period 6 rare earths. Those most commonly used for photostability testing metals are dysprosium, thallium and holmium because of their high color rendering index and high luminous efficacy which means they are good replicators of daylight spectrum, in particular, D65.

There is one lamp which does not fall into this category, the Dr. Honle60 lamp which is a proprietary lamp reportedly based upon iron as the active metal. Little has been published about this lamp, to date, and there is but one manufacturer, Dr. Honle.

The purpose of the halide, usually iodine or bromine, in these and tungsten-halide lamps is to react with the volatilized metal, converting it into a volatile metal halide. This process results in less deposition of the metal on the lamp glass/quartz surface, which if not removed would cause a rapid change in the output of the lamp. Regeneration of both the metal and the halide also occurs when the volatile metal halide collides with the hot electrode. Both of these reactions prolong the life and intensity of the lamp.

Since these lamps are short-arc lamps they are subject to all of the properties of short-arc lamps, such as rapid changes in output and contamination of the emitted SPD by metals volatilized from the electrodes and shorter lifetimes then most other lamps.

As can be seen in Figure 8, taken from the IESNA Handbook, there are several lamps which can be found in this category and one should be careful to select the proper lamp and the proper filter. Eckhart56 gives examples of many other metal halide lamps that have been developed but which are specific for special applications but not applicable to a general pharmaceutical photostabilty test.

9.4 Room tight

Room light is a term, as cited previously, that is undefinable. Many people would consider it to be simply the light provided by a "cool white" fluorescent lamp. As previously discussed and documented in the work performed by Cole and associates,34 large variances can be found in real room light situations.

Current fluorescent lamps, i.e. post 1997 in the US are most likely to be triphosphors, many not having same SPD of the pre-1997 lamps and of a lower intensity because of their thicker phosphor coating. To compensate for this latter item it is not unusual to see many situations where the diffusers, used in the past, have been eliminated, thereby increasing the possibility of exposure to UV-C.

The International Standards Organization (ISO) has developed a standard based on a Toshiba "cool white" fluorescent lamp, ISO 10977:1993(E) "Photography - Processed photographic colour films and paper prints - Methods for measuring image stability". To date I have been unable to confirm that Toshiba does produce a lamp guaranteed to comply with this particular standard. I have also been informed that Toshiba fluorescent lamps are not sold in the US because they are not cost competitive. These items present problems for those who would want to use this particular source.

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