Method of measuring surface of a liquid by illuminating the surface of the liquid

1. A method of measuring a surface of a liquid, the surface of the liquid having an edge which contacts a solid structure and an interior area which does not contact the solid structure, the method comprising:

illuminating a portion of the surface of the liquid and the solid structure with a pattern from an illuminator, the pattern having an upper part and a lower part, wherein the upper part of the pattern is projected onto the solid structure to form an upper projected pattern and the lower part of the pattern is projected onto a part less than all of the interior area of the surface of the liquid to form a lower projected pattern;

acquiring an image of the upper projected pattern; analysing the image of the upper projected pattern to determine a height of the edge of the surface of the liquid;

acquiring an image of the lower projected pattern; and

analysing the image of the lower projected pattern to determine a characteristic of the interior area of the surface of the liquid by determining a spatial frequency characteristic of the image of the lower projected pattern.

2. The method of claim 1 wherein the pattern comprises a set of parallel lines, dots, circles or grids.

3. The method claim 1 wherein the solid structure comprises a wall of a tank which contains the liquid.

4. The method of claim 1 wherein the liquid is a liquid fuel.

5. A method of measuring a surface of a liquid, the surface of the liquid having an edge which contacts a solid structure and an interior area which does not contact the solid structure, the method comprising:

illuminating a portion of the surface of the liquid and the solid structure with a pattern from an illuminator, the pattern having an upper part and a lower part, wherein the upper part of the pattern is projected onto the solid structure to form an upper projected pattern and the lower part of the pattern is projected onto a part less than all of the interior area of the surface of the liquid to form a lower projected pattern;

acquiring an image of the upper projected pattern; analysing the image of the upper projected pattern to determine a height of the edge of the surface of the liquid;

acquiring an image of the lower projected pattern; and

analysing the image of the lower projected pattern to detect a contaminant floating on the surface of the liquid.

6. The method of claim 1 wherein analysing the image of the upper projected pattern to determine a height of the edge of the surface of the liquid comprises detecting discontinuities in the image of the upper projected pattern.

7. The method of claim 1 wherein the pattern is an optical radiation pattern.

8. The method of claim 1 wherein the liquid is carried by a vehicle.

9. The method of claim 8 wherein the vehicle is an aircraft.

10. A method of estimating a volume of a liquid, the method comprising measuring a surface of the liquid by the method of claim 1 ; and estimating the volume of the liquid in accordance with the determined height of the edge of the surface of the liquid.

11. The method of claim 10 wherein the volume of the liquid is estimated in accordance with both the determined height of the edge of the surface of the liquid and the determined characteristic of the image of the lower projected pattern.

12. An apparatus for measuring a surface of a liquid, the surface of the liquid having an edge which contacts a solid structure and an interior area which does not contact the solid structure, the apparatus comprising:

an illuminator arranged to illuminate a portion of the surface of the liquid and the solid structure with a pattern having an upper part and a lower part, wherein the upper part of the pattern is projected onto the solid structure to form an upper projected pattern and the lower part of the pattern is projected onto a part less than all of the interior area of the surface of the liquid to form a lower projected pattern; an image acquisition system arranged to acquire an image of the upper projected pattern and an image of the lower projected pattern; and

a processor programmed to analyse the image of the upper projected pattern to determine a height of the edge of the surface of the liquid, and to either: analyse the image of the lower projected pattern to detect a contaminant floating on the surface of the liquid; or, analyse the image of the lower projected pattern to determine a characteristic of the interior area of the surface of the liquid by determining a spatial frequency characteristic of the image of the lower projected pattern.

13. An aircraft comprising a fuel tank; and apparatus according to claim 12 for measuring a surface of liquid fuel in the fuel tank.

14. The method of claim 5 wherein the pattern comprises a set of parallel lines, dots, circles or grids.

15. The method claim 5 wherein the solid structure comprises a wall of a tank which contains the liquid.

16. The method of claim 5 wherein the liquid is a liquid fuel.

17. The method of claim 5 wherein analysing the image of the upper projected pattern to determine a height of the edge of the surface of the liquid comprises detecting discontinuities in the image of the upper projected pattern.

18. The method of claim 5 wherein the pattern is an optical radiation pattern.

19. The method of claim 5 wherein the liquid is carried by a vehicle.

20. The method of claim 19 wherein the vehicle is an aircraft.