EP2076811A1

EP2076811A1 - Device used in wavelength modulation spectroscopy and system using such a device

Info

Publication number: EP2076811A1
Application number: EP07835065A
Authority: EP
Inventors: Elman Ulf
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-10-02
Filing date: 2007-09-28
Publication date: 2009-07-08
Also published as: SE0602039L; SE529931C2; WO2008041905A1

Abstract

The invention relates to device (1,19,24) arranged for use in a wavelength spectroscopic system. The device is adapted for rotation around a rotational axis, and has a portion transparent to light extending through it. The device is arranged for receiving or is provided with an optical filter (3) in the transparent portion such that the filter normal and the rotational axis are not equal. The transparent portion maybe constituted by a through hole. In a particularly advantageous embodiment, the device simply comprises the inner rotating part (19) of a motor having a through hole in the inner rotating part. The invention further relates to a wavelength modulation spectroscopic system with a device according to the invention. A wavelength modulation spectroscopic system with such a device is a particularly simple design for spectroscopic detection of substances, such as water ice.

Description

Device used in wavelength modulation spectroscopy and system using such a device

The present invention relates to a device used in wavelength modulation spectroscopy (WMS) according to the introductory portions of the independent claim and a system using such a device.

Background of the invention

In wavelength modulation spectroscopy a wavelength selective element having a periodically varying centre wavelength is used. One possible way of implementing such a wavelength selective element with a varying centre wavelength is to periodically pivot the angle of incidence of light transmitted through an optical dielectric filter as the centre wavelength of such dielectric filters shifts with angle of incidence.

Pivoting an object in a back-and-forth movement is however not a very practical solution as it put stringent demands on the mechanical system. A more practical way of achieving a periodically varying centre wavelength has been suggested by S. B. Lyakhov and G. G. Managadze in the article "Modulation photometer with rotating interference filter", Pribory i tekhnika eksperimenta, 18, 200-201 (1975). Here they suggest mounting a dielectric optical filter at the end of a motor shaft, with the filter mounted with its normal at an angle α/2 off the motor shaft axis. Incoming light is then transmitted through the filter at an angle of α/2 off the motor shaft axis. As the motor shaft with the filter rotates, the angle between the filter normal and the incoming light will vary periodically between 0 degrees and α degrees.

To avoid the incoming light hitting the motor, the shaft has to be comparatively long, making it difficult to achieve a directionally stable shaft. The shaft with the filter mounted on it will be prone to wobbling, such that a varying portion of the incoming light will be blocked by the axis. In a worst case, parts of the incoming light may even miss the filter altogether, and pass through the arrangement unfiltered. To prevent light missing the filter or hitting the shaft, only a small portion of the filter area may be used, which is disadvantageous.

Alternatively, only the central portion of the filter is used for transmission, in order to prevent light from passing beside the filter. In this case the shaft blocks part of the light beam, causing a varying transmission through the system. This variation will be synchronous with the rotational frequency, which is precisely the frequency the wavelength modulation spectroscopic system is sensitive to.

US4536057 discloses a holder for an optical element adapted for use in an optical system with an optic axis. The optical system is cylindrically symmetric with the optic axis at its centre. The holder is adapted for receiving an optical element and is arranged to be held in the optical system with the optical element normal collinear with the optic axis.

In a particular embodiment of the invention according to US4536057, the optical element holder is arranged for use with polarizers and, in order to make it possible to set the polarization angle, the holder is turnable at least half a turn. The holder is however not carried in a bearing and is not suited for rapid, continuous rotation, such as necessary for a device as suggested by Lyakhov and Managadze. Further, the holder is arranged for turning around the optic axis, and is not provided with means to receive an optical element with its normal deviating from the optic axis. The suggested embodiment is therefore not useful for achieving the periodically varying angle between the incident light and the optical element normal.

An object of the invention is therefore to provide a device for WMS which overcomes the above- mentioned problems with prior art optical filter arrangements for achieving a periodically varying wavelength.

These and other objects are attained by a device for WMS and a system using such a device according to the characterising portions of the independent claims.

Summary of the invention

The invention relates to device (1, 19, 24) arranged for use in a wavelength spectroscopic system. The device is adapted for rotation around a rotational axis, and has a portion transparent to light extending through it. The device is arranged for receiving or is provided with an optical filter (3) in the transparent portion such that the filter normal and the rotational axis are not equal. The transparent portion may be constituted by a through hole.

In a particularly advantageous embodiment, the device simply comprises the inner rotating part (19) of a motor having a through hole in the inner rotating part. The invention further relates to a wavelength modulation spectroscopic system with a device according to the invention. A WMS system with such a device is a particularly simple design for spectroscopic detection of substances, such as water ice. Such WMS based systems for ice detection is ia. disclosed in US7224453.

Brief description of the drawings

Fig. 1 shows a prior art arrangement.

Fig. 2 shows a first embodiment of the device in a front view.

Fig. 3 shows the first embodiment from the side.

Fig. 4 shows the first embodiment from the side in partial cross section.

Fig. 5 shows a second embodiment of the device in a front view.

Fig. 6 shows the second embodiment from the side in partial cross section.

Fig. 7 shows a third embodiment of the device in a front view.

Fig. 8 shows a fourth embodiment of the device from the side in cross section.

Fig. 9 shows a fifth embodiment of the device.

Description of preferred embodiments

Fig. 1 shows the prior art arrangement suggested by Lyakhov et al. Lyakhov suggests arranging a dielectric filter at the end of a long motor shaft, with the filter normal at an angle to the shaft axis. The light is then transmitted through the filter at an angle to the motor shaft axis. As can be clearly seen, it is difficult to avoid that the motor itself blocks the light beam, and the shaft blocks part of the beam. Lyakhov suggests using only half the filter for transmission of light, avoiding hitting the shaft, which obviously decreases transmission to half of what could have been transmitted through the whole filter. Also, mounting a filter at the end of the shaft poses difficulties, and a viable solution would be to use some attachment device fastened to the filter by an area larger than the tip of the shaft itself, further reducing total transmission through the suggested arrangement. Achieving a directionally fully stable shaft would also be difficult, and a wobbling shaft would cause varying transmission through the arrangement or part of the light passing unfiltered beside the filter.

Fig. 2 shows a first embodiment of the device in a front view comprising two interacting spur gears, an upper filter gear (1) holding a filter (3) and a lower driving gear (2). The filter gear is held in place by three impellers (4a-c) that are arranged around the filter gear and run in a groove (5) along the periphery of the filter gear which is more clearly illustrated in fig. 3. The impellers are journalled on axes (6a-c) allowing them to rotate freely around the axes, while still holding the filter gear in place. The filter gear has a rotational axis around which it is free to rotate, and through the centre of the filter gear runs a though hole in which the filter is arranged with the filter normal at an angle to the rotational axis. As the filter gear rotates, the filter normal will therefore precess around the rotational axis of the filter gear.

Fig. 3 shows the first embodiment from the side, where it is clearly illustrated how the filter gear is held at a distance from a backing plate (7) by the impellers, of which in the illustrated view only two (4a, 4b) are visible. The groove (5) along the periphery of the filter gear, in which the impellers are running, is also visible. On the rear side of the backing plate is a shafted motor (8) mounted, and the driving gear is attached to the shaft of the shafted motor.

Fig. 4 shows the first embodiment from the side in partial cross section, such that the shaft of the shafted motor acting on the driving gear, and the filter mounted in the through hole of the filter gear, are visible. It is more clearly visible in this view how the filter is mounted in the though hole of the filter wheel with the filter normal at an angle to the rotational angle of the filter wheel. Incoming light, illustrated by arrows, is directed towards the filter at an angle from the rotational axis of the filter gear, and light transmitted through the filter is emitted in the same direction. In the position of the filter wheel illustrated in the figure, the angle between incoming light and the filter normal is at a maximum, turning the filter gear half a turn positions the filter with its normal parallel to the direction of the incoming light. Obviously, as the filter wheel is turned by the action of the shafted motor, the angle between the filter normal and the incoming light varies periodically with a period equal to the rotation period of the filter wheel.

The incoming light beam in the figure is illustrated as fully collimated and an optical system, not illustrated, is necessary for selecting only near collimated light passing through the filter at the angle chosen. If the light passing through the filter is not perfectly collimated, as would normally be the case, the effective transmission line width of the system would widen as compared to the special case of fully collimated light beam. In the case illustrated in fig. 4, the angle between the incoming light and the rotational axis equals the angle between the filter normal and the rotational axis, but this does not have to be the case.

The through hole in the filter gear is adapted to receive the filter and hold it in place, while allowing transmission of an as large portion of the light hitting the filter as possible. At the same time the through hole is designed to block all light not hitting the filter, such that only light having passed through the filter is transmitted. These blocking portions are not clearly illustrated in the figure to simplify the illustration only.

Fig. 5 shows a second embodiment of the device in a front view, having a filter gear held in a gear holder (9) and a driving gear (2). The gear holder is designed to receive the filter gear and while allowing it to rotate freely, still holding it in place inside the gear holder. The gear holder has through holes (10) aligned with the through hole of the filter gear, such that a clear aperture through the gear holder and the filter gear is achieved.

Fig. 6 shows the second embodiment from the side in partial cross section, such that the cylindrical cavity (11) in the gear holder which receives and holds the filter gear is visible. A corresponding through hole in the backing plate is shown, such that a clear aperture through the gear holder, filter and backing plate is achieved. The gear holder has a rear opening which is slightly narrower than the filter gear, and part of the middle section (13) of the filter gear is visible. The middle section of the filter gear interacts with a correspondingly narrow driving gear, driven by a shafted motor as in fig. 3 and 4.

Fig. 7 shows a third embodiment of the device in a front view. Here, the driving gear acts on the filter gear with a synchronous drive belt. The filter gear is attached to the inner ring of a ball bearing, while the outer ring of the ball bearing is fixedly mounted.

Fig. 8 shows a fourth embodiment of the device from the side in cross section, where the filter is simply arranged inside the hollow shaft of a direct drive torsion motor (16). By direct drive torsion motor is here meant a motor having a through hole in the centre rotating part (19) of the motor, usually the rotor, instead of a shaft. Obviously, any motor having a through hole in its centre rotating part is covered by this definition. Again, the filter is arranged with its normal at an angle to the rotational axis of the motor, and it is held in place by a holder (17) which also acts to block all light not hitting the filter from being transmitted through the system. The device is intended to receive incoming light at an angle to the rotational axis, as illustrated by the arrows.

Fig. 9 shows a fifth embodiment of the device, having essentially the shape of a cone (24) rotated around its centre axis by a motor (8). The cone (24) is at its bottom (22) wider end truncated orthogonally to the cone centre axis, while the top narrow end is truncated along a plane with its normal at an angle off the cone centre axis. A filter (3) is held in place at an angle to the centre axis of the cone, at the top of the cone. The bottom of the cone is transparent to light passing through the filter and the bottom also receives the shaft of the motor. The cone outer surface (21) connects the bottom with the filter.

Light enters the filter at an angle to the cone centre axis, as illustrated by the arrows, passes through the interior of the cone and exits trough the lower half of the transparent bottom (22) without hitting the motor shaft. The light then hits a mirror (23) which reflects the light away from the motor, allowing the motor shaft to be shorter than it would have to be without the mirror.

In the illustrated embodiment, the cone outer surface (21) is preferably non transparent, to block light not passing through the filter from being transmitted. Alternatively, the cone top may be wider than the filter and non transparent. For such an embodiment, not illustrated, the whole cone may be constituted by a solid transparent block.

Throughout this document references are made to a filter (3), and this should be interpreted as any optical filter having a wavelength dependent transmission, reflection or absorption which changes with the angle of incidence of incoming light. In particular, by filter is meant an optical dielectric interference filter, and in a preferred embodiment an optical dielectric interference band pass filter.

A WMS system using a single specific optical filter mounted in a device like one of the embodiments shown above is only able to detect and identify presence of a small number of substances having interfering spectral properties. Such a set of substances may, i.a. be liquid water and water ice, but any other set of substances may be chosen by selecting the properties of the filter and the device for receiving the filter. Obviously, the WMS system may comprise two or more optical filters, giving it the ability to detect and identify presence of a larger number of substances, or the information from the filters may instead be used for extending the range of substance thicknesses the system may accept.

Although the invention has been described in conjunction with a number of preferred embodiments, it is to be understood that various modifications may still be made without departing from the scope of the invention as defined by the appended claims.

Claims

1 A device (1, 19, 24) arranged for use in a wavelength modulation spectroscopic system with light incident essentially along an optic axis, adapted for rotation around a rotational axis with a first angle separating the optic axis and the rotational axis, where said device is arranged to receive an essentially flat optical filter (3) with a normal, where the filter normal and the rotational axis are separated by a second angle, characterised in that the device is arranged for receiving the essentially flat optical filter (3) in a transparent through hole such that the received filter is enclosed by the device along at least the main part of its periphery.

2 A device according to claim 1, characterised in that the device comprises the inner rotating part (19) of a motor having a transparent portion in the inner rotating part.

3 A wavelength modulation spectroscopic system with light incident essentially along an optic axis, comprising a device (1, 19, 24) adapted for rotation around a rotational axis with a first angle separating the optic axis and the rotational axis, where said device is arranged to receive an essentially flat optical filter (3) with a normal, where the filter normal and the rotational axis are separated by a second angle, characterised in that the device (1, 19, 24) is arranged for receiving the essentially flat optical filter (3) in a transparent through hole such that the received filter is enclosed by the device along at least the main part of its periphery.

4 A wavelength modulation spectroscopic system according to claim 3, characterised in that the device comprises the inner rotating part (19) of a motor having a transparent portion in the inner rotating part.

5 A wavelength modulation spectroscopic system according to claim 4 or 5, characterised in that the system is adapted for detection of at least water ice.