In the daily use of the interferometer, especially in the production workshop, because we can easily use the computer to analyze anytime and anywhere, we usually use the naked eye to judge the quality of the product based on the interference fringe, which is the fastest and most convenient.

    There are two types of interference fringes: the first one is an equal thickness interference fringe, and the light and dark stripes in the equal thickness interference fringes are equally spaced, and each adjacent strip represents the same thickness interval.

Assume that the horizontal line is the standard surface, and the oblique line is a surface to be measured with a fixed slope. When the light hits, the refraction phenomenon will occur. We make a dotted line parallel to the standard surface at the first injection point, and the surface to be tested will be Light a is reflected back, and light b is reflected back on the standard surface. It can be seen from the figure that the paths through which light rays a and b pass are different, and when the optical path difference is exactly an integer multiple of the wavelength, it can be seen. Interference fringes to the same interval

    The second is an isosceles interference fringe, which is an interference fringe formed by rays of the same angle. P1 has an interference fringe whose source is caused by four solid lines, and the four solid pairs The surface of the object is caused by the same angle of light, because the object is circular, so it will cause symmetrical effect. The four dotted lines are caused by light of another angle, and then P2 is generated. An interference fringe of a point. Therefore, the interference fringes formed by the same angle of light are called equal-inclination interference fringes, but in practical applications, equal-thickness interference fringes and isosceles interference fringes may occur simultaneously.

Application: Surface flatness

    If we want to understand the flatness of the surface of the object from the interferogram, we can draw a crosshair centered on the interferogram, counting the number of stripes in the X direction and the Y direction from the center point. There are several stripes, which is the most commonly used in optical factories. When we ask Master to grind a lens, we can tell us the need for surface flatness, the tolerance of the error range in the X and Y directions. how many.

  Suppose that there is 1 stripe in the X direction and 3 strips in the Y direction. That is to say, the component to be tested changes in the X direction and the Y direction. The degree of change is defined as the surface. The uniformity of the surface irregularity is defined as: POWER, which is 3 in the Y direction, and irregularity is the difference between the X direction and the Y direction, that is, 2, so we can use the interference fringe from the above figure. Know that the power of the object to be tested is 3, the irreverity is 2, what exactly is POWER, what is irreregularity?

Suppose that the component we are looking at is the lens of the glasses. When viewed from the side, the lens will focus when there is light, and the degree of focusing will be different for different bending amounts. We call it magnification, and the degree of curvature of the surface is defined. For POWER. The degree of bending in the X and Y directions on the lens may be different, that is to say, POWER is not the same, we call it Surface irregularity,

Now that we know that the interferogram fringe is represented as 3/2, what is the number represented? His unit is the wavelength, generally 632.8nm wavelength, 3/2 3 means 3 wavelengths, 2 means 2 The wavelength is usually expressed in wavelengths in the calculation of optical components.

Note: We use wavelengths as the evaluation unit in interferometer measurements, so we also need to pay attention to the wavelength of the interferometer used. Suppose the same lens, the A manufacturer uses the interferometer with λ=500, the interpretation data is 3/2, the B manufacturer uses the interferometer with λ=600, and the interpretation data is also 3/2, then the A manufacturer using the 500λ interferometer The data must be better, because the shorter the wavelength, the smaller the conversion to data, so in addition to interpreting the data of the interferogram, it is also necessary to pay attention to whether the wavelength used for the interference matches the requirements to get the most correct. result.

Example 1: What can we read from POWER and Irregularity from Figure A?

After adding the cross coordinate, there are 2.5 stripes in the X direction and 1.5 stripes in the Y direction, so the maximum bending amount of this lens is 2.5, and the difference between X and Y is 1, but the result of this interferogram is not 2.5/1

When the X direction is the same as the bending direction of the surface to be tested in the Y direction, the irregularity is subtracted by 2, but when the bending direction of the X direction and the Y direction is different, the irregularity is added.

When the X direction is the same as the bending direction of the surface to be tested in the Y direction, POWER takes the maximum value, but when the X direction and the Y direction are different in the bending direction of the surface to be tested, the POWER is subtracted.

Therefore, the irregularity calculated from this figure is 1.5+2.5=4, and the X direction and the Y direction can be regarded as the same face, so the POWER is 2.5-1.5=1. Therefore, we must first know what object is measured, otherwise The data obtained may also be wrong.

Several common interference fringes:

It should be noted that the interference fringes in these figures are caused by the comparison of the object to be tested and a standard plane. Once the stripe is changed, the resulting strips will all change, and the corresponding conditions will be completely different.

The Without tilt on the left side is: When there is no tilt effect coming in, the stripe changes produced by different surfaces to be tested

The On tilt on the right side is: When the tilt effect comes in, the fringe changes produced by different surfaces to be tested

1 or 2 when the surface to be tested is flat, Without tilt will not see the stripes

When the surface to be tested is curved surface 3, Without tilt will present concentric circles with dense edges and unequal spacing.

When the surface to be tested is spherical 4, Without tilt will present concentric circles with equal spacing.

Assuming that the standard surface is a plane, the shape of the object to be tested may be hyperbolic or ellipsoid, so the thickness changes more severely, and the object to be tested of 4 may be spherical or nearly spherical.

Therefore, when doing interferometer measurement, when you want to judge the shape of the interference fringe, you must first understand the shape of the object to be measured, or judge the shape of the object by the shape of the interference fringe.

    Because the interference fringes will vary with the reference plane, so when I want to know the shape of the surface to be tested, I must first know what the shape of the standard surface is. Now let's take the same shape of the object to be tested - convex lens as an example.

    When the object to be tested is a spherical surface and the reference surface is a standard plane, the interference fringes may be a concentric circle distribution. However, if the reference surface is changed to the spherical surface of the standard curvature, the interference fringes may become a straight line distribution. The reason why the side has different interference fringe distribution is that the interference fringe sees the difference between the side to be referenced and the reference surface. Therefore, if you want to judge which interference fringe the object to be tested is a spherical surface, you must first understand. What is the reference referenced during the measurement? The shape of the surface to be tested can be judged correctly by the interference fringes.