A screen printing method and apparatus are capable of maintaining a constant attack angle between a squeegee and a mask plate, regardless of the surface condition of the mask plate. The display device using a diffractive light modulator includes a light source unit, an illumination unit, a diffractive light modulator, and a projection unit. Other embodiments are described and claimed. An actuator is connected to the compression cylinder for compressing the elastic ring and causing it to grip the probe tip.