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A substrate distortion measurement apparatus comprising one or more optical detectors arranged to measure the locations of pits or holes provided in a substrate, a memory arranged to store previously determined locations of the pits or holes in the substrate, and a comparator arranged to compare the

A substrate distortion measurement apparatus comprising one or more optical detectors arranged to measure the locations of pits or holes provided in a substrate, a memory arranged to store previously determined locations of the pits or holes in the substrate, and a comparator arranged to compare the measured locations of the pits or holes with the previously determined locations of the pits or holes, to determine distortion of the substrate.

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1. A method comprising: (i) measuring the sizes and locations of marks on a substrate;(ii) determining distortions in the substrate by comparing the measured sizes and locations with previously measured sizes and locations of the marks; and(iii) exposing the substrate to patterned radiation,wherein

1. A method comprising: (i) measuring the sizes and locations of marks on a substrate;(ii) determining distortions in the substrate by comparing the measured sizes and locations with previously measured sizes and locations of the marks; and(iii) exposing the substrate to patterned radiation,wherein a first side of the substrate is exposed to the patterned radiation and wherein the sizes and locations of the marks are measured from a second side of the substrate that is opposite to said first side. 2. The method of claim 1, wherein said marks include pits. 3. The method of claim 2, wherein said pits have a width in the range of 100-1000 nm. 4. The method of claim 1, wherein distortions in an area of the substrate are determined prior to exposing the area of the substrate. 5. The method of claim 4, wherein said marks face away from said area of the substrate. 6. The method of claim 4, wherein said patterned radiation is provided using a contrast device. 7. The method of claim 6, further comprising adjusting a pattern of the contrast device relative to the determined distortions in said area of the substrate prior to exposing the area of the substrate to radiation. 8. The method of claim 1, wherein said substrate comprises a plastic layer. 9. The method of claim 1, wherein said substrate comprises a glass layer. 10. The method of claim 1, wherein said measuring the sizes and locations of marks on a substrate is effected using a laser. 11. The method of claim 10, wherein the marks are provided in a layer of the substrate which is opaque and reflective to light at a wavelength used to measure the sizes and locations of the marks, and one or more other layers of the substrate are substantially transparent to the wavelength. 12. The method of claim 1, wherein the distortions in the substrate are determined by comparing the measured sizes and locations with previously measured sizes and locations of the marks along one direction and wherein distortions in the substrate along another direction are determined by directing light onto the marks such that the light is reflected from the substrate towards one, or more detectors, the light being generated by one or more optical sources mounted on a servo, anddetecting movement of the servo when tracking the marks, the detected movement being indicative of distortions of the substrate in said other direction. 13. The method of claim 1, wherein said direction and said other direction are substantially perpendicular to each other. 14. A substrate distortion measurement apparatus comprising: (i) a detector arranged to measure sizes and locations of marks provided on a substrate,(ii) a memory arranged to store previously measured sizes and locations of the marks that are located on the substrate, and(iii) a comparator arranged to compare the measured sizes and locations of the marks with the previously measured sizes and locations of the marks,wherein, in use, a first side of the substrate is exposed to a patterned radiation so as to form a pattern on said first side and wherein the sizes and locations of the marks are measured from a second side of the substrate that is opposite to said first side. 15. An apparatus according to claim 14, wherein one or more optical sources are arranged to direct light onto the substrate such that the light is reflected from the substrate towards the one or more optical detectors, the light being modulated by the marks in the substrate. 16. An apparatus according to claim 15, wherein each of the one or more optical sources is mounted on a servo and is arranged to track marks on the substrate. 17. An apparatus according to claim 16, wherein the servo is arranged to provide an output which indicates movement of the servo when tracking the row of pits on the substrate. 18. An apparatus according to claim 17, wherein the output of the servo is connected to the comparator. 19. An apparatus according to claim 15, wherein an output from each optical detector is connected to the comparator. 20. An apparatus according to claim 14, wherein the comparator is configured to output a signal representative of distortion of the substrate, the signal being based on the comparison between the measured sizes and locations of the marks with the previously measured sizes and locations of the marks. 21. A lithography apparatus comprising: a substrate distortion measurement apparatus comprising: (i) a detector arranged to measure sizes and locations of marks provided on a substrate,(ii) a memory arranged to store previously measured sizes and locations of the marks that are located on the substrate, and(iii) a comparator arranged to compare the measured sizes and locations of the marks with the previously measured sizes and locations of the marks, andan array of moveable mirrors arranged to form a pattern to be projected onto the substrate, and control electronics arranged to adjust the pattern formed by the moveable mirrors to compensate for the determined distortion of the substrate,wherein a first side of the substrate is exposed with the pattern and wherein the sizes and locations of the marks are measured from a second side of the substrate that is opposite to said first side.