我如何知道设备屏幕有多少物理英寸？

I have a website that needs to distinguish between mobile phones, tablets, and ordinary laptops. Normally I could use an asp.net function to tell, but the complication is that I want tablets to be treated as NOT being mobile devices. Any device with a screen diagonal greater than 7.5 inches would get the normal website, and any device with a screen dialog of less than this would get the mobile version of the site.I have a website that needs to distinguish betw