Head-up display, a combiner used for the head-up display and a method of designing the head-up display

A combiner for a head-up display which is disposed on a windshield glass of a vehicle to diffract light carrying information from an information display source toward an observer in the vehicle, the combiner comprising:

• a hologram which has a diffraction spectrum with at least three diffraction peaks, each of said diffraction peaks having a central wavelength .lambda., a half width .DELTA..lambda. and a diffraction efficiency .eta. with respect to the light produced from the information display source and diffracted toward the observer inside the vehicle,
• said sets of central wavelength, half width and diffraction efficiency having values which cause a color difference between the color tone of the light from the information display source and the average color tone of the light reflected from the combiner over the angles of observation to be minimized or which cause the sum of the square of a color difference between the color tone of the light from the information display source and the color tone of the light reflected by the combiner to be minimized, wherein each of the three central wavelengths is within one of the ranges of 580-650 nm, 500-570 nm and 420-490 nm, where the color difference is defined to be a color difference .DELTA.E*.sub.ab according to the L*a*b* color system ruled in JIS-Z8730 and is expressed by .DELTA.E*.sub.ab ={(.DELTA.L*).sup.2 +(.DELTA.a*).sup.2 +(.DELTA.b*).sup.2 }.sup.1/2 where .DELTA.L* is the difference of psychometric lightness and .DELTA.a* and .DELTA.b* are the differences of psychometric chroma coordinates which causes the color tone of the light from outside the vehicle through the combiner to the observer and the color tone of the light reflected by the combiner to the outside of the vehicle to be close to white.

A method of designing a head-up display comprising:

• an information display source, comprising at least a light source for producing light and a display device for displaying information which is displayed by means of the light;
• and
• a combiner arranged on a windshield glass of a vehicle to diffract the light to an observer in the vehicle;
• said combiner including a hologram which has a diffraction spectrum with at least three diffraction peaks each of said diffraction peaks having a central wavelength .lambda., a half width .DELTA..lambda. and a diffraction efficiency .eta. with respect to the light produced from the information display source and diffracted toward the observer inside the vehicle, said sets of central wavelengths, half width and diffraction efficiency having values which cause a color difference between the color tone of the light from the light source and the average color tone of the light reflected from the combiner over the angles of observation to be minimized or which cause the sum of the square of a color difference between the color tone of the light reflected by the combiner to be minimized, wherein each of the three central wavelengths is within one of the ranges of 580-650 nm, 500-570 nm and 420-490 nm, where the color difference is defined to be a color difference .DELTA.E*.sub.ab according to the L*a*b* color system ruled in JIS-Z8730 and is expressed by .DELTA.E*.sub.ab ={(.DELTA.L*).sup.2 +(.DELTA.a*).sup.2 +(.DELTA.b*).sup.2 }.sup.1/2 where .DELTA.L* is the difference of psychometric lightness and .DELTA.a* and .DELTA.b* are the differences of psychometric chroma coordinates which causes the color tone of the light from outside the vehicle through the combiner to the observer and the color tone of the light reflected by the combiner to the outside of the vehicle to be close to white.