High spatial resolution imaging colorimeter and gloss-meter for measurements of small parts

A high spatial resolution imaging colorimeter and glossmeter is presented. The combination of a high quality imaging colorimeter with an off axis imaging optics and a highly stabilized RGB LED source allows color and gloss measurements of small parts with high accuracy. The characteristics of the system with some experimental results on black coated and printed papers are presented. Introduction Surface quality inspection has become important in a variety of fields such as the metal, plastic, paper and printing industries. Optical measurement methods, in particular, are widely used for surface quality inspection since the optical measurements do not destroy the measurement objects [1]. Two commonly used parameters for surface appearance evaluation are surface color and gloss. Conventional gloss-meters and colorimeters can provide such information averaged on a small spot on the surface of a flat object. Nevertheless, these instruments are not capable to measure very small pieces and do not provide any information on the sample microstructure that generally plays a key role in the aspect of the object. The purpose of this paper is to present a new instrument capable to measure color and gloss of small pieces with high spatial resolution and high accuracy. The gloss is strictly speaking the ratio of the reflectance ratio of the measurement with a C source in very specific geometric conditions to the measurement in the same conditions on a black flat glass mirror [2]. In the literature, different methods have been proposed to make gloss imaging but very few fulfill exactly the standards. The simplest setup includes a collimated beam illumination coupled to an imaging camera at the specular position [3]. Combination of linear collimated light source and line-scan camera improves the evenness of focus [4]. Diffractive optical element based gloss-meters have also been introduced [5]. This type of instrument works even on curved surfaces but requires a scanning to make imaging with medium spatial resolution. Imaging colorimetry has been used for many years and various applications. To obtain color accuracy in all conditions, there is no other solution than making a system that closely matches the CIE curves at any wavelength [6]. It is why sensors with imbedded colors filters cannot be used for this purpose. One practical solution is to use monochrome sensors and to match their spectral response to CIE curves adjusting the transmittance of the color filters to each sensor [6]. For imaging of non-emissive objects it is needed to illuminate and the control of the color and stability of the light source are potentially additional sources of errors. In the present paper, we introduce an instrument capable to measure the color and gloss of 3x3cm surfaces with high spatial resolution and excellent accuracy. The key points of the system are presented first. Then several examples of applications are reported. Experimental details General description: A schematic diagram of the system is presented in figure 1. It is composed of an off-axis imaging objective associated with a high resolution ELDIM UMaster video colorimeter, a computer control LED based illumination and a mechanical setup for easy control of small samples. The sample is illuminated at several geometric configurations and observed at 25° in specular configuration. It is mounted on a horizontal sample holder that can be shift back for easy replacement. Vertical movement of the illumination and detection allow manual adjustment of the focus. All the components are included in a box to get rid of the external parasitic light. Figure 1. Schematic diagram of the VGC100 system Imaging colorimeter Imaging polarimeter UMaster is based on a Peltier cooled CCD sensor with true 16-bit analog digital converter. UMaster includes a set of five color filters designed specifically for each CCD sensor. These filters are manufactured as a combination of different color glasses that work in absorbance. Advantages are very good accuracy because of the adaptation of the design to each CCD sensor and an excellent durability. All the objectives are telecentric on the sensor side to ensure the same transmittance of the filters in the entire field of view of the system. In addition this configuration ensures that collection efficiency is independent of the distance to the object [7]. Off axis imaging optics For the present measurement system, the objective is also telecentric on the object side to ensure the light collection on the entire field of view (cf. figure 2). In addition the imaging optics is slightly tilted in order to obtain a well-focused image in the entire field of view for a collection angle of 25°. An intermediate iris is used to fix the collection aperture that is the same for all the point on the object surface (~4° in the present configuration). Figure 2. Schematic diagram of the off axis imaging objective and large aperture illumination in specular of diffused configuration Illumination lamp The LED illumination is realized with RGB LEDs regularly positioned on a dedicated IC that can be controlled with a USB connection. Additional RGB photodiodes and temperature sensors allow a good stabilization of the emission. The source emission is calibrated in an absolute way using a reference spectrophotometer [8]. The independent control of the R, G, and B channels allows adjustment of the color of the illumination within the RGB triangle (cf. figure 3). Two light source configurations are available for gloss or color measurements. For gloss measurements, the LED source is included in an integration sphere whose exit is used as punctual source with the same telecentric optic to obtain a collimated beam illumination in specular configuration. The illumination aperture is maintained constant all over the field of view and much smaller than the collection aperture (~1°). For color measurements, a flat diffusor on top of the LED IC increases the homogeneity of the source. The maximum luminance for white state is around 1400cd/m. Two positions are possible for the source, one in specular configuration and one in backscattered configuration (cf. figure 2). Figure 3. Emission spectra of the R, G and B LEDs of the LEDLamp and