Metal detection has two different components: permeability and conductivity. Although the most important component may be conductivity, most foods will leave a "fingerprint" that combines these two elements. Metals also show an obvious combination of the two effects. For example, most ferrous metals show stronger magnetic permeability, while non-ferrous metals show higher electrical conductivity. On the other hand, stainless steel indicators are easily overwhelmed by product effects.

When using the X-ray machine, light waves will pass through the product to be detected. Therefore, products with different densities will affect the performance of X-ray. To illustrate this point, take a box of grain as an example. Although the X-ray machine will be calibrated according to the total density of the product, for example, the density of grain composition, raisins and bran will be different. The density of raisins will be high and the bran will be low. Because X-rays are measuring the aggregation density, if there are pollutants in the box and the machine falls on low-density grain, it may not be able to detect the sample as it falls on high-density raisins.