## 1 Introduction  
In the 1990s, [PERSON] et al. demonstrated that femtosecond laser pulses are useful to induce refractive index change within bulk glasses [1]. Ultrafast lasers have become a powerful tool for material writing photonic structures in widely varying glasses over the years [2; 3]. In femtosecond laser micromachining, structural changes of different kinds are induced in the bulk glass. Modification types include refractive index change [1; 4; 5; 6], birefringence [7; 8], color center [9], and scattering damage [5; 10]. For applications to femtosecond laser micromachining, investigation of the morphology and discrimination of structural changes is important [11].  
Generally, researchers check their written structures with optical microscopes under different illumination configurations in a daily and routine manner. In bright-field imaging, the illumination numerical aperture (NA) must be matched to the collection NA by adjusting the condenser diaphragm size. In dark-field imaging, an aperture stop is placed at the condenser diaphragm to ensure that the illumination NA is larger than the collection NA. Conventional optical microscopy such as phase-contrast microscopy and differential interference contrast (DIC) microscopy can enhance contrast in refractive index change. In phase-contrast microscopy, a ring aperture is placed at the condenser diaphragm to match the ring-shaped phase plate of the objective lens. Phase-contrast microscopy is useful for the observation of refractive index change in the glass to enhance the contrast of modifications [12; 13] because the refractive index change is small (typically on the order of \(10^{-2}\)-\(10^{-3}\)[1]). These observation methods require replacement of optical elements: a special objective lens for phase-contrast imaging and an iris diagram. Rheinberg illumination microscopy enables simultaneous acquisition of bright-field and dark-field images using two-color illumination [14; 15]. The central passing region of the condenser filter is used for bright field illumination. The annular zone is used for dark field illumination.  
Active illumination sources, such as an LED array, programmable condenser lenses with a projector illumination, and projector-pattern illumination have provided acquisition of bright-field, dark-field, and differential phase-contrast (DPC) images by various illumination patterns of light sources. We demonstrated that multi-contrast images of structural modifications in the glass were obtained using a digital light processing (DLP) projector [16]. In the DLP-based microscope, the illumination pattern of the DLP projector must be projected onto the ground glass screen. Then, the light scattered at the ground glass acts as a secondary light source. Then, the scattered light is focused by a condenser lens onto the sample. Therefore, the system becomes large. In fact, LED array microscopy is easy to implement with conventional microscopy; bright-field, and dark-field images are obtained by changing illumination patterns of the LED array without changing the objective lens or the iris diagram [17]. In fact, LED array microscopy is categorized into two regimes: time-coded and color-coded. In time-coded LED array microscopy, multi-contrast images such as bright-field, dark-field, and differential phase contrast (DPC) are acquired by temporally changing the illumination patterns of the light source [18]. In color-coded LED array microscopy, three color-coded illumination allows simultaneous acquisition of bright-field, dark-field, and DPC images at a single shot [19].  
This report describes the acquisition of a multi-contrast image of structural modifications produced in glass using an LED array microscope. We also present simultaneous acquisition of bright-field and dark-field images of structural modifications in glass with LED-array-based Rheinberg illumination.