Some filters are made with a thin gel or even colored glue laminate sandwiched between two pieces of regular clear glass, similar to the glass used in windowpanes. These types of filters are cheap to produce, but inferior for several reasons:
To make colored filters, Hoya adds different raw elements, like gold, and chemicals to its optical glass while it is in a molten state. This insures that Hoya filter glass is uniformly colored all the way through. There is never any risk of uneven coloration, shifting or fading of the color, or delamination. The two surfaces are ground and polished for perfect flatness.
Hoya manufactures a full line of filters in both standard and Hoya multi-coated. The difference between Hoya's standard line and that of other manufacturers is that Hoya standard filters have one layer of anti-reflective coating applied to each surface of the glass. Many other manufacturers standard filters are bare glass, and bare glass can reflect as much as 9% of the light hitting it. This greatly increases the risks of flare and ghosting.
This can be seen in a simple test. Take a bare glass filter hold it so that light reflection off the surface can be seen. Then take a long very thin object like a pin or the tip of a pen and hold it over the filter so that its reflection can be seen. There will actually be two reflections of the pin on the surface, one a little more pronounced than the other. The more pronounced reflection is from the front surface and the lighter one is from light reflecting off the rear surface.
Hoya's single layer coating decreases light reflection off the surface from approx. 9% to an average of 4-5%.
To provide photographers with a higher quality professionals require, Hoya created the Multi-coated line of filters. These filters have a 3 layer coating system that further reduces light reflections off the surfaces of the glass, the average is only 1-2%. This means that 98-99% of the light striking the filter is going through it, and depending on the type of filter, into the camera lens and onto the film. These layers of anti-reflective coating are bonded to the surface of the glass in a furnace at a temperature of up to 800 degrees F.
You should beware! Some other manufacturers claim to have "coated" filters. But this coating is often only applied to the front side of the glass, not both sides like Hoya filters. Also, the coating on many filters is "painted" on or applied as a cold spray that wears off easily.
The wide aperture lenses of today are very expensive and all photographers want to get the most speed, optical performance, and dollar performance from their investment.
Say a customer pays $500.00 for a 28-70mm f/2.8 lens. Then, to protect this investment the customer buys a cheap bare glass filter, which has a light reflection rate of 9%. This filter is literally slowing the lens down by 9%, or effectively turning a $500 f/2.8 lens into the equivalent of a slower f/3.0 lens worth $455. The value of the lens drops 9% when you put the cheap filter on it. The cost savings of the less expensive filter do not off set the loss of lens speed.
Also, this does not address the loss of sharpness or focus shift, which can have a noticeable detrimental impact on picture quality. For these reasons, Hoya multi-coated filters present the best value on the market today.