An image display device that includes: a fluorescent screen in which stripe-shaped regions, in which phosphors that are excited by light to emit fluorescent light are formed, are repeatedly formed in the in-plane direction; and a scanning system that scans the fluorescent screen with excitation light is known. Phosphors include a phosphor that emits red fluorescent light, a phosphor that emits green fluorescent light, and a phosphor that emits blue fluorescent light. These color phosphor regions are repeatedly formed in a predetermined order on the fluorescent screen.
Generally, in an image display device of this type, the relative positional relationship between the scanning system and fluorescent screen typically undergoes change due to various causes such as vibrations or distortion, changes in the environment such as in temperature or humidity, the effect of gravity, or changes that occur with the passage of time. In addition, if the scanning system is a resonance mirror or the like, the temperature of the resonance mirror that reflects excitation light that has been intensity-modulated varies depending on the intensity of the excitation light. As a result, the resonance frequency of the resonance mirror also varies. If the resonance frequency of the resonance mirror varies, the phase and amplitude of the scanning system vary and thus the scanning position deviates from the correct position.
If a static variation and a dynamic variation such as variation of the relative positional relationship between the scanning system and the phosphor screen or a variation of the scanning position occurs, stripe-shaped or matrix-shaped color phosphor regions cannot be irradiated with the excitation light at an appropriate timing. As a result, the luminance of fluorescent light which the individual color phosphor regions emit vary and it causes deterioration of color purity of a displayed image.
To solve such a problem, the positions of the individual color phosphor regions on the fluorescent screen need to be accurately detected and the individual color phosphor regions need to be irradiated with excitation light at an appropriate timing.
Patent Literature 1 describes an image display device that can control the irradiation timing at which phosphor regions are irradiated with excitation light.
The image display device described in Patent Literature 1 has a light source, a fluorescent screen, a deflection unit that scans the fluorescent screen with excitation light emitted from the light source, a half mirror located in the traveling direction of the excitation light directed from the deflection device, a photo detector, and a drive circuit that controls the light emission timing at which the light source emits light based on the output signal of the photo detector.
The fluorescent screen has individual color (red, green, and blue) stripe-shaped visible fluorescent phosphors that are repeatedly formed at predetermined intervals in the in-plane direction and stripe-shaped dark lanes formed adjacent to individual visible fluorescent phosphors. Stripe-shaped reflection means made of a cube mirror is formed at every second dark lane. The reflection means reflects incident light in the opposite direction of the direction of the incident light.
The fluorescent screen is irradiated with the excitation light directed from the deflection unit through the half mirror. The light emission timing is controlled in such a manner that the fluorescent screen is scanned with a predetermined quantity of excitation light. When the fluorescent screen is scanned, the individual color visible fluorescent phosphors and the reflection means are irradiated with the excitation light. The excitation light with which the reflection means is irradiated becomes retro-reflection light that travels in the opposite direction of the direction of the incident light. The retro-reflection light reaches the half mirror. Part of the retro-reflection light reflects on the half mirror and then enters the photo detector.
The output signal of the photo detector is supplied to the drive circuit as an index signal that serves to detect the positions of the individual visible fluorescent phosphors. The drive circuit predicts the positions of the individual color visible fluorescent phosphors based on the output signal of the photo detector and controls the light emission timing of the light source such that the visible fluorescent phosphors are irradiated with the excitation light at an appropriate timing.