Why our vision gets blurred, if we look at the spark that comes while welding?

The simple explanation to this effect is the temporary lapse in recognizing the intensity between the bright and the dull area of the seen viewed, which occur due to incomparable higher intensity of spark (light) from the welding. The brain recognizes the picture by the matric form built by the difference in light intensities over different areas and the difference in the frequencies of light or colour.
Biologists explain it, if we look at spark like spots, the higher light causes a light induced stress i.e. photo oxidation stress in the retina called, short term stress, which causes the blurred vision. But after some time the stress gets released thus restoring normality. Viewing high intensity light like sunlight and ionizing radiation for a long time can damage the retina leading to vision loss. That is why welding professionals are advised to use protective filter glasses, biologists advises.