GB2194269A - Optical window - Google Patents

Abstract

A window to protect optical apparatus on the outside of vehicles includes a first portion (11) of hemispherical shape formed from a material transparent to optical radiation and of constant thickness. A second portion of the window is of toroidal shape formed integral with the first portion. It has an inner surface (20) substantially in the form of a cylinder the axis of which passes through the centre (12) of the hemisphere, and an outer surface (21) which in contrast to the prior art is formed such that the thickness of the material increases gradually away from the hemisphere. The second portion is thus formed so as to have the same optical effect upon radiation passing through it as does the first portion of the window. <IMAGE>

Description

SPECIFICATION Optical window This invention relates to optical windows, and in particular to optical domes used to protect optical apparatus carried on the outside of vehicles. It is common practice to make such windows from a plastics material such as polycarbonate or polymethylmethacylate, though other materials may be used, and high quality windows giving a field of view extending up to a hemisphere are readily moulded. However, if the window is required to extend beyond a hemisphere, to give a wider field of view, then problems arise. In order to continue with the spherical form very complex moulding tools have to be used which are formed in several parts. Using such tools it is difficult to avoid lines or other blemishes where parts of the tool join.A dome of uniform thickness acts as a negatively-powered lens, the power of which depends upon the curvature, thickness and refractive index of the material. Hence any fault which causes variations in these parameters causes optical aberrations. Removing faults, if possible at all, is very expensive.

It is an object of the invention to provide an optical window of such a form that it may be moulded with simple tools without the risk of faults of the type referred to.

According to the present invention there is provided a window for optical apparatus which comprises a first portion of hemispherical shape formed from a material transparent to optical radiation and of constant thickness, and a second portion of toroidal shape formed integral with the first portion and having an inner surface substantially in the form of a cylinder the axis of which passes through the centre of the hemisphere and having an outer surface formed such that the thickness of the material increases gradually away from the hemisphere, the second portion of the window being formed so as to have the same optical effect upon radiation passing through it as does the first portion of the window.

The term "cylinder" is used in this specification to cover not qnly an exact cylinder but also a slightly frusto conical surface in which the apex angle is not more than 10 .

The invention will now be described wih reference to the accompanying drawings, in which: Figure 1 illustrates a common form of optical window; Figure 2 illustrates a first embodiment of the invention; and Figure 3 illustrates the method of determining the profile of the optical window of Fig. 2.

Hemispherical windows, or domes, are wellknown. These are easily moulded by simple tools as there is no part of the window which would inhibit withdrawal of the inner part of the moulding tool. If a piece of optical apparatus is located inside the window then its field of view is limited, at best, to a hemispherical solid angle. In practice this is often too restricted and a window giving a wider field of view is required. For the reasons already given it is not possible to continue the spherical shape to form a bowl-shaped window. Fig. 1 shows one solution which may appear to be satisfactory but which, in practice, gives rise to further problems. The major portion of the window below a diameter 10 the window comprises a hemispherical shell 11 having a centre 12. The remaining portion of the window above the line 10 the surface is substantially cylindrical as shown at 13.The surface 13 departs, in practice, from a true cylinder by a small angle 14, shown greatly exaggerated in Fig. 1. This angle is usually necessary to ensure easy withdrawal of the inner part of the moulding tool, and may be up to a maximum of 5". The surface 11 will be slightly less than a hemisphere to accommodate the angle 14.

The problem with a simple window as shown in Fig. 1 is that whilst the hemispherical portion acts as a spherical lens, focussing incoming radiation to a point, the cylindrical portion acts as a cylindrical lens and focusses incoming radiation to a line. Clearly this is not acceptable.

Fig. 2 illustrates a form of window which overcomes the problem. As will be seen from the drawing, whilst the inner surface 20 of the upper portion of the window remains substantially cylindrical as before, the shape of the outer surface 21 is curved gradually away from the hemispherical oportion, producing an increase in thickness of the material of the window. The form of the outer surface 20 is such that the upper portion acts as a spherical lens of the same power as the hemispherical portion of the window.

There is a limit to the form of the curve of surface 21 as the thickness of the material must not be allowed to become too large since this affects the power of the lens so produced.

There is no simple mathematical definition of the shape of the surface 21, and this has to be determined rather by empirical means.

Fig. 3 illustrates one method of determining the shape. The figure is drawn assuming that parallel rays of light R from outside the window pass through the window to the inside to become diverging rays of light R'. Incident rays R1 to R4 may be drawn passing through the hemispherical portion of the window. Using the expression sin i/sin r=,u and knowing, from the drawing, both i and ,, it is possible to draw the corresponding refracted rays R1' to R4'.

Since the upper portion of the window is required to act as a lens of the same power as the lens formed by the hemispherical por tion of the window, the refracted rays R6' to R9' may be drawn as mirror images about the line 10 passing through the centre of the hemisphere-. Since ju will be the same and the value of r at the surface 20 can be measured from the drawing, it is now possible to determine the shape of the outer surface 21 which produces rays R6 to R9 parallel to rays R1 to R4 and which provides a surface continuous with the outer surface of the hemispherical portion 11.

Whilst the above description has considered moulded windows made from a plastics material, the form of the window and the way in which this is determined is equally applicable to windows formed in other ways, such as by machining or vapour deposition, or formed from other materials.

Claims (5)

1. A window for optical apparatus which comprises a first portion of hemispherical shape formed from a material transparent to optical radiation and of constant thickness, and a second portion of toroidal shape formed integral with- the first portion and having an inner surface substantially in the form of a cylinder the axis of which passes through the centre of the hemisphere and having an outer surface formed such- that the thickness of the material increases gradually away from the hemisphere, the second portion of the window being formed so as to have the same optical effect upon radiation passing through it as does the first portion of the window.

2. A window as claimed in Claim 1 in which the toroidal portion extends through an angle of at least 15 from the hemispherical portion as measured from the centre of the hemispherical portion.

3. A window as claimed in either of Claims 1 or 2 which is formed from a plastics material.

4. A window as claimed in any one of the preceding claims in which the inner surface of the second portion is inclined inwards towards the first portion by an angle not exceeding 5".

5. An optical window substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.