Review on 🔪 Warner 4-inch Big Blade Scraper with 5-inch Steel Handle - Model 691 by Christian Young

I use this blade for checking optics - whatever you say. it's the old knife edge trick. If you place a knife edge (commercially KE) over a small light source and set the optical surface under test to reflect back over the source and look at the reflection in KE, you will see the 1st derivative of the optical surface and from the They determine if the optical surface is smooth and then apply the optical Focault test invented in 1858. So I needed a long KE and these scraper blades have worked brilliantly for this purpose - they need to be sharpened the same way you sharpen a razor blade. With this simple blade and some experience, you can test the optical surface to within a tenth of a light wave. Now you know how valuable KE is.