The tolerance for surface accuracy is represented by the maximum allowable sagittal deviation, irregularity, and rotational symmetry irregularity. Meanwhile, the root mean square (RMS, including RMSt, RMSi, RMSa) is often used to indicate surface flatness, too.

1. Identification Method

Under ISO 10110-5/ GB/T 2831-2009 standard, there are the following three forms: 3/A (B/C); 3/A (B/C) RMSx<D;  3/RMSx<D (x represents t, i, a).

The above “3” stands for the code for surface accuracy. A is the sagittal deviation, B means the irregularity (IRR), C indicates the rotational symmetry irregularity (RSI), RMSt is the root mean square total deviation, RMSi the root mean square irregularity, and RMSa the root mean square asymmetry.

 

1)     A (SAG): The maximum allowable difference between the curvature radius of the measured optical surface and that of the reference surface, with one aperture corresponding to 1/2 lambda.

 

2)     B (IRR): The maximum allowable irregularity value, or the corresponding aperture number representing the astigmatism deviation.

 

3)     C (RSI): The maximum allowable rotational symmetry irregularity, or the aperture number referring to the local deviation.

 

4)     RMSt: Maximum allowable root mean square total deviation, indicating the root mean square deviation between the inspected optical surface and the nominal theoretical sphere.

 

5)     RMSi: Maximum allowable root mean square irregularity, meaning the root mean square discrepancy between the tested optical surface and the fitted sphere.

 

6)  RMSa: Maximum allowable root mean square asymmetry, referring to the root mean square gap between the measured optical surface and the fitted aspherical surface.

 

2. N, ΔN, and PV, RMS.

In actual production, Photonchina uses test plate or lens templates, and Zygo interferometer in the workshop to control the surface accuracy of optical parts. N, ΔN are common surface parameters for spherical optical components.

When using a standard optical template for inspection, surface indication includes three items: N (Radius deviation), Δ1N (astigmatism deviation), and Δ2N (partial deviation).

N – Radius deviation: The curvature difference between the inspected optical surface and the reference surface, which generates a certain number of apertures.

 

Δ1N – Astigmatic deviation: The inconsistent curvature radius of the measured optical surface in the mutually perpendicular direction relative to that of the reference optical surface, which is reflected in the unequal number of interference fringes in the mutually perpendicular direction.

 

Δ2N – Partial deviation: The deviation of a certain area of the measured optical surface relative to the surface of the reference optical surface produces partially irregular interference fringes in some directions.

Δ2N=e/H. e represents the amount of the deviation between partially irregular interference fringes and ideal smooth fringes, while H referring to the spacing between adjacent fringes.

 

For an interferometer inspection, the PV value and RMS value are only provided within the actual inspection area.

The PV value corresponds to the gap between the peak and valley values of the wavefront, and it is not very comprehensive if solely based on PV value. RMS is a statistical measurement that looks to the whole changes in the wavefront, and the RMS value therefore reflects the wavefront situation in a more objective way. See the drawing below,

 

 

 

3. Relationship between PV, Power and N, ΔN.

The PV of the actual inspected wavefront has the following two meanings,

1)     The gap between the highest and lowest points of the wavefront;

2)     The PV value after wavefront removal (subtraction) from the best fitting sphere.

 

The Power value is generally set to be N, and the PV value after removing the Power value is ΔN.

a、     When the surface being inspected is good and the Power value is equal to or approximately equal to PV, then the PV value or Power is equal to N.

b、     When the measured surface is not good or underperformed, the Power value is usually small, so the PV value is directly taken as ΔN. In fact, at this point, the value of N is relatively meaningless, only ΔN is required.

c、     When the quality of the tested surface is average, the Power value is still N, and ΔN cannot obtain accurate results. Approximately, the result of subtracting the Power value from the PV value could be seen as a reference for ΔN.

 

 4. Relationship between PV,RMS, Power and ABC values.

The output parameters of interferometers generally provide three values: PV, RMS, and Power, in addition to interferograms, two-dimensional graphics, and three-dimensional contour maps.

The ZYGO interferometer output interface has an ISO 10110-5 window, which displays the PV, RMS, and Power values. The instrument will provide SAG, IRR, RSI, corresponding to A, B, and C values, respectively.

 

5. Zygo Inspection Report

Photonchina can provide Zygo laser interferometer inspection reports that may include PV data, PVr data (Zernike fitting), 3D, XY axis profile curves, etc. depending on the request of customers. Please see the following example for an acylindrical lens with 50*25mm.