OP.DI.MA. Diffuse material
ODM (or OP.DI.MA.) is a special processed synthetic material which works as a translucent reflector. This means that radiation will enter into the material and reflect from many interfaces within the material itself. This results in a nearly perfect diffuse reflection. A side effect of volume reflection is a variation in transit times of the reflected radiation within the material. This variation in transit eliminates speckling effects associated with coherent radiation.
10 mm thick ODM98 provides a reflectance of up to 98.5% in the visible spectrum and a minimum reflectance of 93% within the wavelength range from 250 nm to 2.5 µm. Other properties of OP.DI.MA. include temperature stability up to 280° C, insolubility in water and resistance to UV degradation. The surface of the material must be machined to obtain an optical finish without gloss effects. It can be re-machined and cleaned prolonging its operational life. Of course, these outstanding properties are accompanied by some limitations: the material can only be manufactured in block or plate form. This generally necessitates performing cutting operations to shape the material to the desired form. As a translucent reflector, the level of reflection depends on the material thickness. The material can not be applied as a coating. So successful use of ODM requires correct implementation and processing strategy.
1. Reflectance of ≥ 10mm thick Material:
ODM98 is the purest form of OP.DI.MA. offering the highest possible reflectance within the UV-VIS-NIR wavelength range. In ≥10mm thickness a reflectance of 98 +/- 1% is specified in the visible wavelength range from 400 to 800 nm, whereas a minimum reflectance of 93 % is specified within the entire useful wavelength range from 250 nm to 2.5 µm. See picture 1 and 2 in the graphical specifications.
2. Reflectance of < 10mm thick Material:
Translucent reflectors reflect light within the material itself. As a result reflectance is thickness dependent. The thinner the material the lower the reflectance allowing more light to pass through the material (see picture 3 in the graphical specifications). For visible light about 70% reflectance is achieved within the first few hundred micrometers of the material. In some applications the recommended 10 mm thick material cannot be used.
Typical examples where 10mm thick material cannot be used are:
- Reflectors made out of ODM98 foils (thin sheets)
- Machined knife-edge integrating sphere ports
- Diffuser Windows
3. Surface Influence on Reflectance:
Reflectance level and diffuse reflectance quality depends on material properties, surface conditions and thickness. Any mechanical abrasion of the surface may result in gloss reflections. Also, absorption of volatile substances alters ODM98’s reflective behavior. The adverse optical effects caused by surface contamination like grease (fingerprints) or oil are similar to those of other optical components. If the surface cannot be cleaned using distilled water (without pressure), material must be removed.
4. Ageing Characteristics:
ODM98’s optical stability under exposure to high power optical radiation irradiation is one deciding factor for its selection over other reflectance materials. Its fundamental long term stability results from the basic material itself, in association with a careful manufacturing process employing ISO 9001 certified Quality Management and has no real limitations. Quantitative data on UV resistance is only provisional, because of the many factors which actively influence practical applications. One test, as shown in the adjacent graph (picture 4 in the graphical specifications) shows an ageing test on an ODM98 reflection standard with a 420 W halogen-metal vapor lamp operating in the 250 to 400 nm wavelength range. Field reports involving applications using high-powered laser cavities and on-line fluorescence sensors confirm minimal ageing under exposure to UV.
5. Maximum Permissible Radiation Flux Density:
The maximum permissible radiation flux density for ODM98 can, like many other parameters, only be described through particular examples. In a series of tests an ODM98 reflection standard was exposed to pulsed laser radiation. The wavelength of the laser was 1064 nm, the focus diameter 200 µm, the pulse length 200 µs, the repetition rate 25 Hz and the mean power was 21 W. The peak power of 4.2 kW resulting from this, representing a radiation flux density of 13.3 GW/m2 did not yield any detectable alterations. Surface impurities were removed by the laser itself at the start of the test series.
6. Temperature Characteristic:
ODM98 is capable of operating in high temperatures up to 280° C. This allows the material to be used in intense applications environments. Investigations of the reflection factor dependency on operating temperature indicate that it is highly stable. Picture 5 in the graphical specifications shows that the reflection factor during temperature cycles from room temperature up to +100° C remains stable. In fact the measured deviation was within the repetition accuracy of the experimental set-up used. Using temperature cycles up to +170° C constant change in the reflectance of about 1% has been recorded. This is due to tempering processes occurring within the material at high operation temperatures. Of course, at increasing temperatures, reduction in mechanical stability must be considered.
Machining OP.DI.MA. is not difficult for those with some experience with processing plastics. All machining methods such as lathe work, drilling, sawing, milling, etc., are possible. See Block Material for Customer Processing for more information.
ODM98 is chemically inert, and is stable against acids, bases and other organic solvents. A special manufacturing process also gives it high mechanical stability. These properties mean that dust, for instance, can be removed with paper tissues (lint-free, no pressure). Other impurities can be removed with water/alcohol mixtures or with methylene chloride or hexane (hexane is appropriate for organic impurities). When cleaning with solvents, these penetrate into the micro-porous (amorphous) structure of the plastic. After cleaning, which can, for instance, be carried out using solvent-soaked paper tissues, the plastic must be ”baked” at a temperature near the solvent’s boiling point. Scratches and ”polished” areas which diminish the reflective properties can be removed by removing the top layer with fine-grained abrasive paper. It is recommended that calibrated reflection standards be re-calibrated after cleaning.