What Is a Stroboscopy?

Strobe is to look at the screen of another appliance in the screen of another appliance. The screen of another appliance will move a bright line from the bottom of the screen to the top, and then appear from the bottom. This infinity goes on, giving us the impression that the image is flickering. In the absolute sense, no strobe is practically impossible, and the Chinese industry standard requires a strobe of 3125Hz for qualified products.

Strobe

The display principle of the display. The dynamic image we see on the monitor is actually composed of a static image, and there is a slight difference between two adjacent images. Each image is called a frame, and it switches to the next frame after being displayed for a short time. The transition between the two frames is full and extremely short, but it is fully prepared to display the next image. Because the human eye has a visual persistence phenomenon, the images that are seen will not disappear immediately, so the frames seen are coherent.

There is a display called CRT, which is a "cathode ray display". The working principle is this. A high-temperature electron generating device (called an electron gun) emits a large number of electron beams, and a strong magnetic field is used to control the deflection of the electron beams. Highlights. The image processing device tells the strong magnetic field which points should emit light and which points should not emit light.

As mentioned earlier, a dynamic picture is made up of a series of slightly different frames. When displaying a frame, instead of displaying the entire image at once, the electron gun scans the screen points one by one according to a certain path. It is usually swept down line by line. The display process is as follows: first it is completely black for a short time, and then it is scanned line by line until all the lines are scanned, one frame of image display is completed, and then it is completely black to prepare for displaying the next image. So at some point, a frame of image may have just ended, it may have just begun, it may be half displayed, or it may be completely black.

The camera is also shooting one by one still image, with little difference between frames. Changes in a very short time may not be captured, we can think of it as a high-speed camera.

The computer monitor can display more than 60 images per second (this ability is called the refresh rate), which is assumed to be 70 here, and the camera is generally 24 frames per second. That is to say, the computer has completed several frames of display during the shooting of an image by the camera. Suppose that the camera starts to capture the first image at the moment when the computer starts a new frame, then after 1/24 second, the computer displays 70/24 = 3-2 / 24 frames, that is, the three frames are just a little behind. According to the introduction above, we can know that there are still few lines on the computer display that have not been scanned, that is, the bottom lines are still stubborn, and the top is already glowing, so the top is glowing three times, and the bottom is only glowing twice. The resulting image is bright above and dark below, there is a boundary line, the upper bright and the lower dark, just at 22/24. After another 22/24 seconds, this line will move 2/22 above, and then appear at the bottom after moving to the top. This infinity goes on, giving us the impression that the image is flickering. It can be inferred that when the refresh rate is 69 or even 68 miles, flicker will become severe.

The root cause of flicker is that the two appliances have different refresh frequencies.

The strobe time of electrical appliances exceeds the reflection time of nerves in the human eye, so the naked eye looks smooth on both the computer and the TV.

The LCD screen is also stroboscopic. After all, it is an image created by powering up. The optimal refresh frequency of each LCD monitor is different. It can be adjusted to the most comfortable eyes. Although it is different from the CRT principle, there is a backlight board in the middle. The excessive boost of the backlight board will cause flicker.

What is stroboscopic photography?

Flash strobe photography, also known as continuous flash photography, is the continuous flashing of an electronic flash with the help of an electronic flash to record the continuous motion of a moving body on a single screen. Strobe photography is generally divided into two categories from the equipment: one is to shoot with an automatic frequency electronic flash (electronic strobe), and the other is to shoot with an ordinary flash group. Due to different shooting tools and methods, the effects of stroboscopic photos are also unique. Let's first look at the method of taking strobe photos with electronic strobe lights. Electronic strobe light is a new type of photographic lighting fixture. When this kind of light is fully charged, it can flash frequently one after another like a pistol. The flash frequency of the high-frequency electronic strobe light can be adjusted as required. The higher the flash frequency, the more negatives the film is exposed, and the more images appear on the photo. Generally speaking, this kind of lamp can flash dozens or even hundreds of times per second. When shooting a moving body with an electronic strobe, dozens of overlapping, staggered images can be left on the screen. These images generated at regular intervals can give people a strong sense of rhythm. It can make people feel novel because the visual effect of solidifying the scene on a picture with high-speed frequencies is more than usual. Cannot be seen. A black background should be selected for shooting strobe photos with electronic strobe lights, and the subject should be as far away from the background as possible to avoid stroboscopic illumination of the background, which will affect the effect of the subject. If the subject is relatively close to the background, it is best to make the flash and the subject form an illumination angle of side light and side backlight, and the subject should also have obvious contour lines, so as to prevent the image and background tones from being mixed. If you do not have a strobe flash, you can use the following techniques to shoot strobe pictures with ordinary flashes: First, use multiple flashes to form a group of light sources to take stroboscopic photos. Since the shooting interval of the flash group cannot be very close, continuous flashing with an ordinary flash group can only produce a few complete and relatively clear images, and the images generally do not overlap with each other, and the motion and rhythm are relatively poor. When shooting, you must first determine how many images are ready to be left on the screen, how far apart the images are from each other, whether the images should be connected with ghost images, etc., and then determine the position and number of flashes. The second step is to choose a dark background. It is usually better to shoot at night, use a low angle to shoot upwards, take the dark sky as the background, or find a spacious room to ensure that the image of the moving object is not disturbed. In addition, in order to make the picture tidy and the subject protrude, it is better to turn off the other light sources except the flash for main lighting. If you are shooting people's movements, in order to make people figure out the direction of movement, you can place a small amount of very dark lights on the scene, the light source should be as close to the ground as possible, and the subject will be backlit. During the official shooting, open the B door of the camera, lock it with the shutter cable, make the camera's shutter open, and then let the moving body start to move. The photographer manipulates the flash with his hand and flashes in succession according to the different movement posture of the moving body. If you need to take three images, use three flashes to fire three times in succession; if you need to take six images, you can use three flashes to fire two times each, or you can use six flashes to fire six times. It is best to find multiple assistants to operate the flash separately. After reaching a tacit understanding, the effect is also ideal. If the moving speed of the moving body is relatively slow and the movement process is long, then a single ordinary flash can be used for shooting. This strobe method is to fully charge the flash and lock the camera's B door to flash the flash at the moment the moving body starts to record the first image; when the next shape is entered, flash again and record the second Image; in this way, after multiple flashes, multiple moving images of different postures will be left on a photo. How should the exposure of stroboscopic photography be mastered? If several images are separated from each other, the exposure is calculated based on the usual single-light flash. If several images are closely connected and partially overlap, the exposure of these images will increase as the number of flashes increases. Therefore, when overlapping flashes, the exposure should be reduced by 1-2 stops. In order to provide a large range of motion for the moving body, and at the same time to facilitate the movement of each stage of the moving body, it is best to use a wide-angle lens when shooting. In order to be more confident, it is best to conduct several experiments before shooting, and let the moving body or person practice several times within the determined shooting range, so that both sides of the shooting can cooperate with each other, so that the shooting can be sure.

On March 15, 2017, CCTV

(1) Headache and eye fatigue: Many migraine sufferers are very sensitive to light, such as bright light and flicker, it is more likely to cause migraine. Slow blinking is usually more likely to cause migraines than fast blinking.

(2) Photosensitive epilepsy: People with light sensitivity will experience epilepsy even under short-term exposure under visible light modulation in the range of 3-70HZ, which will affect about 1/4000 people aged 5-24, usually Onset occurs around puberty, and 75% of the population is sensitive to light for life.

(3) decreased vision

(4) Distracted: The flicker around the field of view of the human eye is more sensitive to strobes. For example, the strobe lights or the rapid modulation generated by the rear lights of the car will attract the driver to keep watching. Moving eyes in advance may be dangerous to the driver or people and things on the road. The stroboscopic light source combined with the running machine will produce an opening and closing effect, and the harm to humans will result in significantly different moving and stopping speeds.

(5) Autism: Children with autism are particularly sensitive to environmental changes, and strobes in lighting increase the recurrence of this behavior.

In the absolute sense, no strobe is practically impossible, and the Chinese industry standard requires a strobe of 3125Hz for qualified products.

For many "eye-protection desk lamps" without flicker, under normal circumstances, we cannot detect whether there is "strobe" with naked eyes, but we can use the mobile phone to detect it smartly, that is, turn on the camera of the mobile phone and point at the LED lamp. If the frequency flickers, it proves that this LED lamp may have a serious "strobe" problem.