Optical probe heads

Details Optical probe heads Optical probe heads

1993-10-25

1995-08-08

Hannaher, Constantine

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

2503412, 356300, G01N 2155, G01N 2131

Patent

active

054401267

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to probe heads and, in particular, to probe heads utilizing the mechanism of attenuated total reflectance for use in infra-red spectroscopy.
2. Description of Related Art
Attenuated total Reflectance (ATR) crystals use the total internal reflectance to effect an interaction between a sample and beams of radiation of wavelengths suitable for infra-red spectroscopy.
Total internal reflection occurs when the crystal material is optically denser (i.e. has a greater refractive index) than the surrounding medium, provided that the angle of incidence .alpha..sub.1 fulfills a certain condition.


BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be particularly described with reference to the accompanying drawings, in which:
FIG. 1 is an explanatory diagram;
FIG. 2 is a series of graphs showing the variation of penetration depth with incident angle, refractive index and wavelength;
FIG. 3 is a diagrammatic representation of a section through an ATR head in accordance with a specific embodiment of the invention; and
FIG. 4 shows one form of apparatus envisaged by the invention.


DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is an explanatory diagram which shows radiation incident at an angle .alpha..sub.1 on an interface between an attenuated total reflectance crystal of refractive index n.sub.1 and a sample of refractive index n.sub.2. The penetration of the evanescent wave at an interface is given by ##EQU1## provided that sin.alpha..sub.1 >n.sub.2
.sub.1. If sin.alpha..sub.1 .ltoreq.n.sub.2
.sub.1, then total internal reflection does not take place. Instead the ray is transmitted out of the crystal in accordance with Snell's law (n.sub.1 sin.alpha..sub.1 =n.sub.2 sin.alpha..sub.2).
Typical samples may have indices in the region 1.3-1.5. For these, the minimum interface angle which will produce an acceptable penetration depth is dependent on the material used for the ATR crystal. Conventionally, high refractive index substances are used, such as zinc selenide, zinc sulphide, silicon or germanium. However, the mechanical and toxic qualities of these materials make them unsuitable for use in probe heads.
Referring now to FIG. 2 of the drawings, a wide variety of different graphs can be constructed showing the variation of penetration depth with wavelength, angle, refractive indices etc. The series of plots shows the penetration depths obtained in sapphire against the refractive index of a sample under test for different values of .alpha..sub.1, n.sub.1 and .lambda..
Sapphire possesses superior mechanical properties and is non-toxic. However, the minimum interface angle which will produce an acceptable penetration depth is 65.degree.. Below this angle, only samples with very low indices will exhibit total internal reflection.
For angles 65.degree. and above total internal reflection may occur. The penetration depth, however, is such that several reflections (at least 4) are necessary to produce overall pathlengths large enough for good quality spectra.
Due to these requirements, traditional ATR crystal shapes are not suitable for sapphire.
The internal angles of an octagon are 135.degree.. If a light ray is reflected internally between faces, this corresponds to .alpha..sub.1 =67.5.degree.. Thus suitable sapphire ATR crystals may be based on the geometry of an octagon.
A four-reflection crystal would accommodate parallel input and output beams. However, these will necessarily be very close to the sides of the crystal, making the mounting of both the launching optics and the crystal itself very difficult unless a very large crystal is used. We have found that this problem may be overcome by using a five-reflection crystal, desirably, with chamfered input and output faces.
According to the present invention there is provided a probe head for use in attenuated total reflectance optical spectroscopy incorporating a prism having surfaces adapted to provide at least five interfaces with a sample of a material under test at which s

REFERENCES:
patent: 3591287 (1971-07-01), Rodsey
patent: 3669545 (1972-06-01), Gilby
patent: 3751672 (1973-08-01), Michel et al.
patent: 4595833 (1986-06-01), Sting
patent: 4602869 (1986-07-01), Harrick

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