Circular window pieces 

Circular window pieces are commonly used components in optical systems, and their optical parameters are crucial for the performance of the system. The following is a detailed introduction: 

Basic Physical Parameters

Diameter: The diameter of a circular window piece is a key dimension. Common specifications range from a few millimeters to hundreds of millimeters, such as 10 mm, 25.4 mm, 50 mm, etc. It needs to be precisely selected and manufactured according to the specific design and installation requirements of the optical system. The tolerance is usually controlled within the range of ±0.01 mm to ±0.1 mm.

Thickness: Thickness affects the mechanical strength and optical performance of the window piece. A thicker window piece has higher mechanical strength but may increase absorption and scattering, while a thinner one has the opposite characteristics. Common thicknesses are 1 mm, 2 mm, 3 mm, etc. The thickness tolerance is generally required to be between ±0.01 mm and ±0.1 mm.

Thickness Uniformity: It refers to the thickness difference at different positions of the window piece. Good thickness uniformity is crucial for ensuring the consistency of light transmission. Generally, the deviation of thickness uniformity is required to be within the range of ±0.001 mm to ±0.01 mm.

Optical Performance Parameters

Refractive Index of the Material: Different optical materials have different refractive indices. For example, the refractive index of common optical glass like BK7 glass is approximately 1.517 (at a wavelength of 589.3 nm), and that of fused silica is about 1.458 (at a wavelength of 589.3 nm). The refractive index affects the propagation speed and refraction angle of light in the window piece.

Abbe Number: Also known as the dispersion coefficient, it is used to measure the dispersion degree of the material. For instance, the Abbe number of BK7 glass is approximately 64.17. The larger the Abbe number, the smaller the dispersion of the material, which is beneficial for reducing chromatic aberration.

Transmittance: It represents the proportion of light transmitted through the window piece and is related to factors such as absorption and scattering of the material. The transmittance of high - quality circular window pieces can reach over 99% within a specific wavelength range. For example, in the visible light band (400 - 700 nm), the transmittance of some window pieces treated with anti - reflection coatings can reach 99.5%.

Absorptance: It refers to the proportion of light energy absorbed by the material. The lower the absorptance, the higher the transmittance. Generally, the absorptance of high - quality optical materials in their applicable wavelength bands should be less than 1%.

Scattering Rate: It is the proportion of light scattered inside or on the surface of the window piece. Scattering will reduce the imaging quality of the optical system. The scattering rate is usually required to be controlled at a very low level, such as less than 0.1%.

Surface Characteristic Parameters

Surface Figure Accuracy: It reflects the flatness of the window piece surface and is usually represented by the number of interference fringes, such as λ/4, λ/10, etc. (where λ is the wavelength of the laser used for detection, commonly 632.8 nm). λ/4 means that the surface error does not exceed 1/4 of the wavelength.

Surface Roughness: It describes the microscopic unevenness of the surface, and the unit is usually nanometers (nm). Generally, the surface roughness is required to reach Ra 0.1 - 1 nm to reduce light scattering and reflection losses.

Surface Quality: It is usually measured by the scratch - dig standard. For example, 40 - 20 means that the width of scratches does not exceed 40 μm and the diameter of pits does not exceed 20 μm.

Angle - related Parameters

Parallelism: For circular window pieces with two parallel surfaces, parallelism refers to the degree of parallelism between the two surfaces. It is generally expressed in seconds ("). The common requirement is between 10" and 60".

Perpendicularity: It refers to the degree of perpendicularity between the window piece surface and the installation plane or the optical axis. Perpendicularity errors will affect the propagation direction of light. Usually, the perpendicularity error is required to be within a few minutes.