1. A security device comprising:
a homogeneous zero-order diffractive microstructure to display a colour change on tilting or rotating the homogenous zero-order diffractive microstructure, the homogenous zero-order diffractive microstructure being buried within a substrate and comprising spaced lines of material, with spacing between the lines in a range of 100 nm to 900 nm, and wherein the zero-order diffractive microstructure has an index of refraction higher than an index of refraction of the substrate, the zero-order diffractive microstructure configured to act as a waveguide; and
a further structure comprising a plurality of spherical or aspherical micro-lenses formed on a surface of the substrate over the homogeneous zero-order diffractive microstructure, the plurality of micro-lenses being sufficiently small that when viewed by a user an average of different colours will be observed, and wherein the further structure is configured such that the plurality of micro-lenses reduces the colour change produced by the zero-order microstructure upon the device being tilted.
2. A security device as claimed in claim 1 comprising a second further structure formed on a rear surface of the substrate.
3. A security device as claimed in claim 2 in which the second further structure is reflective.
4. A security device as in claim 1 in which a second zero-order diffractive microstructure is buried within the substrate.
5. A security device as claimed in claim 1 in which the further structure comprises a colour filter applied to the surface of the substrate viewed by the user.
6. A security device as claimed in claim 1 further comprising a colour filter embedded in the substrate.
7. A security device as claimed in claim 1 in which the further structure comprises a material embedded in or applied to the surface of the substrate, which material may be locally optically modified by irradiating the surface of the substrate.
8. A security device as claimed in claim 1 in which the further structure is formed by applying a surface treatment to the substrate surface to be viewed by a viewer.
9. A security device as claimed in claim 1, in which the micro-lenses are of size greater than 2 \u03bcm.
10. A security device as claimed in claim 1, in which the plurality of micro-lenses is laterally structured to form a serial number, a one- or two-dimensional barcode, or an image.
11. A method of manufacturing a security device comprising the steps of:
forming a homogeneous zero-order diffractive microstructure to display a colour change on tilting or rotating the homogenous zero-order diffractive microstructure, the homogenous zero-order diffractive microstructure being buried within a substrate and comprising spaced lines of material, with spacing between the lines in a range of 100 nm to 900 nm, and wherein the zero-order diffractive microstructure has an index of refraction higher than an index of refraction of the substrate, the zero-order diffractive microstructure configured to act as a waveguide; and
forming a further structure comprising a plurality of spherical or aspherical micro-lenses on a surface of the substrate over the homogeneous zero-order diffractive microstructure, the plurality of micro-lenses being sufficiently small that an average of different colours will be observed, and wherein the further structure is configured such that the plurality of micro-lenses reduces the colour change produced by the zero-order microstructure upon the device being tilted as viewed at the surface of the substrate by a user.
12. A method as claimed in claim 11 in which the plurality of micro-lenses is formed by ink-jet printing.
13. A method as claimed in claim 11 in which the plurality of micro-lenses is formed by embossing.
14. A method as claimed in claim 11 in which forming the further structure comprises providing a colour filter on the surface of the substrate viewed by the user.
15. A method as claimed in claim 11 in which the further structure is formed by selectively abrading the surface.
16. A method as claimed in claim 11 further comprising the steps of:
embedding an optically modifiable material in the substrate; and
locally optically modifying the material.
17. A method as claimed in claim 16 further comprising the step of irradiating the optically modifiable material to modify an optical property thereof.
18. A method as claimed in claim 17 in which a laser is used to modify an optical property.
19. A method as claimed in claim 11 comprising the step of forming a second further structure behind the zero-order diffractive microstructure.
20. A method as claimed in claim 19 in which the second further structure is reflective.
21. A method as claimed in claim 11, in which the micro-lenses are of size greater than 2 \u03bcm.
22. A method as claimed in Claim 11 in which the plurality of micro-lenses is formed by a wet coating process.
23. A method as claimed in claim 11 in which the plurality of micro-lenses is formed by a roll coating process.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A printing apparatus comprising:
a print head which ejects ink;
a first storage tank which stores ink;
a second storage tank which stores ink supplied from the first storage tank and to be supplied to the print head;
a supply path which forms a portion of an ink circulation path configured to circulate ink among the first storage tank, the second storage tank and the print head, the supply path connecting the first storage tank and the print head via the second storage tank;
a collecting path which forms a portion of the ink circulation path and connects the print head and the first storage tank;
a first pump which is provided in the collecting path between the print head and the first storage tank, wherein the first pump is a sole flow control element in the collecting path, causes ink stored in the second storage tank to be supplied to the print head and causes ink to be directly supplied from the print head to the first storage tank;
a second pump which is provided in the supply path between the first storage tank and the second storage tank, wherein the second pump is a sole flow control element in the supply path and causes ink to be directly supplied from the first storage tank to the second storage tank;
a detecting unit which detects an amount of ink stored in the second storage tank; and
a control unit which controls driving of the first pump and the second pump,
wherein the control unit is configured to drive the first pump at a constant speed when the print head performs a printing operation, increase a driving speed of the second pump, if the detecting unit detects that the amount of ink is lower than a first threshold value, and reduce the driving speed of the second pump, if the detecting unit detects that the amount of ink is higher than a second threshold value greater than the first threshold value.
2. The printing apparatus according to claim 1, wherein the detecting unit has an axis fixed to the second storage tank, and a higher float and lower float through which the axis extends and which are movable vertically in a predetermined range.
3. The printing apparatus according to claim 1, further comprising an ink tank which stores ink to be supplied to the first storage tank.
4. The printing apparatus according to claim 3, further comprising a supply pump for supplying ink from the ink tank to the first storage tank, wherein the control unit controls the driving of the supply pump.
5. The printing apparatus according to claim 4, further comprising a second detecting unit which detects the amount of ink stored in the first storage tank, wherein the control unit drives the supply pump on the basis of the detection result by the second detecting unit to supply ink from the ink tank to the first storage tank.
6. The printing apparatus according to claim 1, wherein the first storage tank is provided at a higher position than the print head and the second storage tank.
7. The printing apparatus according to claim 1, wherein the capacity of the first storage tank is larger than the capacity of the second storage tank.