GB2251939A - Portable spectrophotometer using a doublet dispersive element formed by a transmissive grating and a focusing lens - Google Patents

Abstract

A portable spectrophotometer, for on site study of the absorption spectrum of a substance, uses a doublet (D) formed by a transmissive grating (R) and a focusing lens (L3) as a dispersive element. The dispersed light is detected by a multiple solid state detector system (M, 2, p1-pn), sensing the multiple spectrum elements (F1-fn). This spectrophotometer arrangement is less sensitive to an aggressive or corrosive atmosphere. <IMAGE>

Description

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SPECIFICATION

Portable spectrophotometer for on site study of the absorption SDectrum of a substance The present invention concerns a portable spectrophotometer for on site study of the absorption spectrum of a substance, for example a liquid whose composition is to be studied.

Known spectrophotometers usually employ a dispersive optical system comprising a collimator consisting of a lens (preferably an achromatic doublet) at whose focus is a slit illuminated by the radiation to be analysed. The waves leaving the lens impinge on a dispersive component (prism, grating, etc) which deflects them differently according to their wavelength. The deflected plane waves are then focused to vield in a detection area a succession of images of the source which constitute the spectrum of the radiation. This spectrum is then analysed using an opto-electronic system.

In modern spectrophotometers the dispersive components are usually gratings which provide much higher resolving power than prisms.

Designers have quickly changed over to reflective gratings rather than transmissive gratings which have a narrower spectrum and which cause aberrations because the light passes through them. These aberrations are the result of defects in the composition and the homogeneity of the material through which the light passes and its absorption properties.

)s When they are concave, these gratings have the further 2 advantage of making it possible to dispense with at least one of the two lenses used in thespectrophotometer, namely the lens used to render parallel the rays from the source of radiation and/or the lens imaging at a f inite distance the diffraction spectra formed at infinity.

Although these reflective grating spectrophotometers are excellent laboratory instruments, they are not well suited to the production of portable instruments for carrying out analyses on site, for example analyses to determine the amount of certain pollutants in water.

The reflective grating is usually manufactured by depositing metal in a vacuum, is costly, very fragile and extremely sensitive to pollution by dust and moisture and deteriorates very quickly, in particular by oxidation, corrosion or wear.

Also, these spectrophotometers use opto-electronic systems involving a mobile detector adapted to scan across the spectrum produced by the grating. These systems therefore employ precision mechanical devices offering the appropriate kinematic characteristics and this is hardly compatible with the concept of a portable instrument for use on site under difficult conditions.

A particular object of the invention is therefore to eliminate these drawbacks so as to provide an inexpensive portable spectrophotometer that is nevertheless rugged and reliable, insensitive to moisture or an aggressive or corrosive atmosphere and which does not comprise any moving parts or reflective surfaces that can be degraded by corrosion.

The invention stems from the observation that in the Z 3 context of analyses to be carried out on site using a portable instrument, to detect certain pollutants in water, for example, it is not necessary to scan as wide a range of wavelengths as that of a conventional laboratory spec trophotometer so that it is possible to depart from the conventional design of the spectrophotometers currently used.

Consequently, the invention proposes a spec trophotometer in which the dispersive element consists in a doublet comprising a plane transmissive grating contiguous with a focusing lens and which comprises an optoelectronic solid state detection system employing a multiplicity of detection ranges disposed in the detection area (which covers the area in which the images generated by the doublet are formed).

The detection system advantageously comprises a multislit mask disposed in the detection area and a multiplicity of opto-electronic cells respectively associated with the slits.

Embodiments of the invention will be described hereinafter by way of nonlimiting example and with reference to the appended drawings in which:

- figure 1 is a schematic representation of a spectrophotometer using a spherical lens; 1 - figure 2 is view similar to that of figure 1 but in the case where the focal area of the lens employed is substantially plane; - f igure 3 is a diagram representing in an XOY plane the locus of the focal lengths for a 4 spherical lens doublet of the type used in the spectrophotometer shown in figure 1.

in- both examples the spectrophotometer employs a tank 1 designed to contain the liquid whose absorption spectrum is to be obtained and which comprises a wall transparent at two opposite positions T1, T2 at least.

The tank 1 is illuminated by a halogen light source S through a convergent lens L, designed to form the image of the filament of the source S at the centre of the tank 1 (half way between the transparent sides T1, T2) This image is reproduced at the entry slit F of a spectrograph by means of a focusing lens L2 on the side T2 of the tank 1.

This spectrograph essentially comprises a doublet D comprising a planoconvex collimator lens L 3 with its plane surface contiguous with a plane transmissive grating R oriented so that its etched surface lies against the lens L3 and is therefore protected by the latter.

The luminous image of the filament of the source S at the entry slit F is focused by the lens L3 and deflected by the grating R -according to the wavelengths of the radiation constituting it.

1 Instead of producing a single image of the slit F, as would occur if the grating R were eliminated, a multiplicity of images are obtained spread according to their different wavelengths in an image focal area Z, this succession of images constituting the spectrum to be analysed.

In the example shown in figure 1, the lens L3 is a planospherical lens so that the image focus area (the locus of the images of the slit F for the various wavelengths) is curved.

Figure 3 shows the plot of a focal area of a doublet D whose Crown planospherical lens has a radius of curvature of 32 mm and an axial thickness of 3 mm, this doublet being at a distance of 95 mm from the lens and inclined 7.9' to the optical axis.

In this figure the oval curve C, represents the locus of the focal length of the lens L3 for a mean refractive index n = 600 (Rowland circle - curve resulting from the effect of illuminating a circular optical component off its resolution axis and from the variation of the index of refraction of the lens material).

The curve C2 represents the locus of the images of the slit F for radiation wavelengths between 400 and 800 nm.

In the example shown in figure 1, each slit flr... 1 fn of a curved multislit mask M in the image focal area represents one line of the spectrum to be analysed. The f11... ' fn is channelled by optical fibres 2 connected to photodiodes P11 --- Pn forming part of an electronic analyser circuit A.

radiation received bv these slits It is clear that the invention is not limited to this single solution.

Thus, if a wide bandwidth (10 nm or greater) is acceptable, for some geometrical arrangements the focal area Z' is approximately planar (geometrical position of 6 the doublet, angle of incidence of the light rays).

In this case, the focal area Z' could be on the plane sensitive surface of a strip B of photodiodesforming part of the electronic analyser circuit A (figure 2).

Similarly, it would be possible to insert between the slit F and the doublet D a lens L4 (shown in dashed outline) to collimate at infinity the image of the slit F and to turn the doublet D over (grating R on the side of the lens L4), the lens W3 being then used to direct the images deflected by the grating into the focal area Z'.

As previously mentioned, an important advantage of the spec trophotometers previously described is that they are totally solid state and comprise no moving parts, so that all their optical and opto- electronic components can be accommodated in a totally sealed enclosure. Also, the transmissive grating R is protected by the adjacent lens L3 This eliminates problems of aggression, corrosion and deterioration of operation due to external agencies.

Also, the energy consumption of the spectrophotometer can be reduced so as to increase its working life, in particular by turning taking measurements. In this case a bichromatic measurement is ca ' rried out using the intensity of a line as a reference element to measure the intensity of the required spectral lines.

on the light source S only when i 1 7

Claims (7)

1. Portable spectrophotometer f or the on site study of the absorption spectrum of a substance comprising a dispersive optical element which deflects waves from a source that have passed through said substance according to their wavelength so as to constitute in a detection area a succession of images of the source which constitute said absorption spectrum, characterized in that the dispersive optical element consists in a doublet (D) comprising a transmissive grating (R) contiguous with a focusing lens (L3) and further comprising an optoelectronic solid state detector system comprising a multiplicity of detection ranges located in the detection area (Z).

2. Spectrophotometer according to claim 1, characterized in that the detector system comprises a mask (M) with multiple slits (fl to fn) disposed in the detection area (Z) and a multiplicity of opto-electronic cells (p, to Pn) respectively associated with the slits (fItO fn).

3. Spectrophotometer according to claim 1 or claim 2, characterized in that the lens (L3) of the doublet (D) has a spherical surf ace and in that, in this case, the mask (M) with multiple slits (fl to fn) is curved.

4. Spectrophotometer according to any one of the preceding claims, characterized in that the optical connection between the slits (f 1 to f n) and the optoelectronic cells (P1 to Pn) is provided by optical fibres (2).

5. Spectrophotometer according to claim 1, characterized in that the geometric position of the doublet (D) and the angle of incidence of the light rays impinging on the doublet are calculated to obtain a substantially plane detection area and in that said detection area is 8 represented by the sensitive surf ace of a strip (P) of photodiodes forming part of an electronic analyser circuit.

6. Spectrophotometer according to any one of the preceding claims, characterized in that it comprises a tank (1) illuminated by a light source (S) through a convergent lens adapted to form an image of the filament of the source (S) at the centre of the tank (1), in that t he image is reproduced at a slit (F) by means of a focusing lens ((L2), and in that the doublet (D) is disposed on the axis of the slit (F) so as to generate in an image f ocal area (Z) a succession of images of the slit (F) which constitute the required spectrum.

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7. Spectrophotometer substantially as hereinbefore described with reference to the accompanying drawings.