Optical crown glass

Fused silica (UV grade) has excellent transmittance properties and is both durable and thermally stable. It finds uses as the material of choice for prisms, lenses, windows and cuvettes in the region 160-2000 nm. However, infrared grade fused silica may be used from 250 to 3500 nm. Optical crown glasses are more restricted in wavelength range from 300 to 2500 run. 

Crown Glass. Glass of uneven thickness and slightly convex (thus producing some optical distortion), hand-made by blowing and spinning (cf. optical CROWN glass). 

Optical Crown Glass. Any glass of low dispersion used for optical equipment (cf. flint glass). There are many varieties, their names indicating their characteristic composition, e.g. barium crown, borosilicate crown, fluor-crown, phosphate crown, zinc crown. 

Fig. 1. Frequency of longitudinal phonons (upper curve, left scale) in optical crown glass following 3 temperature change from 330.4 to 331.2 K (lower curve, right scale).

Fig. 4. Comparison of refractive index values at room temperature for optical crown glass (Kronglas BK-7) and Spektrosil A quartz obtained from the literature (L), from measurements with an Abbe refractomer (R), and from BS measurements using the scattering geometries 90A combined with 90N (1), 90R (2), and 180 (3).

The expression (T) can be evaluated from the frequencies of the longitudinally polarized sound modes as well as from the transversely polarized ones. The accuracy of die refractive index determination by BS is comparable to that of refractometric methods. A comparison at room temperature is made in Figure 4 for optical crown glass BK-7 and for Spectrosil-A quartz. 

The earlier types of optical glasses which were melted from the traditionally employed oxides, exhibited approximately direct proportionality between refractive index and dispersion. Deviation from this rule has arisen by the introduction of new components (B2O3, BaO, ZnO, LajOj, etc.)and of quite new types of glasses. The main types and their optical properties are shown in Fig. 145. The current optical crowns and flints occupy a comparatively small region compared with that corresponding to the attainable properties of special optical glasses. 

Normal achromats are made of a crown glass and a flint glass lens. Unfortunately, flint glass becomes nontransparent below about 360 run. Standard achromats therefore cannot be used in the UV. UV achromats can be made of fused silica and calcium fluorite. Their high price and low f numbers make them less useful for TCSPC optics. 

Crown glass 9%Na20 11%K20 5%Ca0 75%Si02 Low refractive index Optical components... 

Figure 5. Uncracked and cracked Knoop indentations in the BK 7 borosilicate crown glass at 19.6 N. Indentations should be spaced further apart for most testing purposes in order to avoid interference, (a) is a bright field and (b) dark field optical microscope illumination. The dark field photo reveals that some localized cracking occurs under the left indentations, but the extent of cracking is dramatically different. The cracked indentation is 7 pjn longer.

Fig. 11. Index of refraction vs dispersion and optical classification of glasses. The shaded area indicates the region of glass formation. BaF = barium flint BaK = barium crown BaLF = light barium flint BaSF = heavy barium flint BK = borosilicate crown F = flint FK = fluorcrown K = crown ...

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