The uniformity of refractive index is quite important for many optical applications of quartz. It is known that not uniformity can be easy detected by so called shadow-method described below in paragraph 3. Now many companies worldwide declared that they are capable to grow optical quartz but actually they can grow only low bubble density synthetic quartz without any requirements on striae density (not uniform optically). To get really striae-free and bubble-free quartz it is important not only to grow quartz in direction down (one side growth) but to have also special chemical composition of water solution with ultra-high stability of temperature gradient in autoclave within 500 days and to use special seeds (without inner stress).
1. As-grown synthetic quartz image
Synthetic quartz stone could be prequalified before lumbering for availability of striae by review the morphology of as-grown Z-surface of stone. Striae-free stone don’t have high hills on Z-surface. But striae (if available) is visible on Z-surface as high hill if compare with low height hills of striae-free area.
2. Dimensions for lumbered blocks of Mineral Ltd.
along Z axis – 1) from 20 to 30 mm and 2) from 31 to 40 mm
along X axis – 1) from 40 to 60 mm and 2) from 60 to 180 mm
3. Measurement method for detection of striae.
The samples for testing can be cut into any form, however the inspected surface should be grinded to R (~ 20μm) or polished (~0.05 μm). If immersion liquid (for grinded or cut surfaces or samples as prisms or lenses) is used its refrection index should not differ from that of the sample more than 2·10 -3
A specially designed projector (see drawing below) is used for detecting the striae of optical quartz. The light source should provide illumination not less than 5 lux at the distance 8000 mm from the diaphragm with diameter 2 mm. Condenser projects the image light on the diaphragm hole. Special glass light filters are used for mitigation of light intensity. The screen receives the shade picture of the striaes and is made of paper covered by chalk fine powder. If photographs are needed special photographic paper is used instead.
Mineral Ltd. apply 4 categories of striae density which are detected by using a set of control samples with different optical grades. Visual comparison allows to define the category of each particular sample. A similar procedure is applied for determination of optical homogeneity.
Mineral Ltd. manufactures and supplies optical quartz with striae categories I, II, III (shade picture for all 3 categories are shown on the picture below).
Below is the picture which shows a shade picture of sample category IV widely used for Optical Low Passband Filters production from one side grown quartz (frequently by mistake named optical quartz by other quartz makers).
Mineral Ltd. offer quartz blocks without striae made according customer’s dimension specification.