There are a TIG (Tungsten Inert Gas Welding) method, a MIG (Metal Inert Gas Welding) method, a MAG (Metal Active Gas Welding) method, etc. in arc welding, for example. Moreover, there area CO2 gas arc welding method (MAG) etc.
The TIG method is a method of generating an arc between a tungsten electrode and a base material in the inert gas atmosphere, such as argon or a helium, and welding by melting a base material and a wedding rod (or welding filler rod) using this arc heat. The tungsten electrode only generates the arc, and does not perform melting shift itself.
In the MAG method, when the welding is started, a welding wire is provided continuously, the arc generated between the welding wire and the base material is continued, and then the welding proceeds. The welding wire is an electrode for generating the arc, and also is melted itself and forms a welding metal with the generated arc heat. In this case, although the welding metal is covered and the atmospheric bad influence is prevented with the shielding gas which flows out from a nozzle of a welding torch tip region, CO2 gas, gaseous argon, and gas which mixes the gaseous argon to the CO2 gas, etc. are used as the shielding gas. The term of the MAG welding is defined in consideration of the kind and characteristics of shielding gas, and the principle as the welding method is the same as the MIG welding or the CO2 gas arc welding.
Although the principle of the MIG method is the same as the MAG method, the gas using inert gas (inactive gas), such as argon and a helium (or these mixed gas), or performed light doping of active gas (activated gas), such as CO2 gas and oxygen, to the inert gas is used as the kind of shielding gas.
Since the arc of the arc welding generally emits a strong light, in particular ultraviolet radiation and visible light, much keratitis and conjunctivitis have been occurred by the ultraviolet radiation in the shop floor. Moreover, an example of retinopathy by visible light is also reported. Although a welding arc emits light to wide wavelength range of ultra-violet, visible, and infrared simultaneously, generally the shape of its spectrum changes with a welding method or conditions.
Also in each the method of the TIG method, the MIG method, and the MAG method, in a conventional welding observation apparatus, there is a problem that an optical dynamic range is narrow and it is difficult to observe simultaneously between the high luminance part of weld and the dark parts of the peripheral part of weld, at the time of the welding.
In a conventional welding observation apparatus using an electronic image synthesis etc. in order to solve the above-mentioned problem, the apparatus becomes large and complicated, is hard to be attached, and is expensive.
In a conventional welding observation apparatus using high-luminance lighting in order to solve the above-mentioned problem, such as laser illumination, the apparatus is large, is hard to be attached, and is expensive, and there is a problem that the arc part is unobservable since it cannot colorize with laser illumination.
In a conventional welding observation apparatus using an ND (Neutral Density) filter in order to solve the above-mentioned problem, there is a problem that the dark part of the peripheral part of weld is unobservable since the color image cannot obtain vividly.
In a conventional welding observation apparatus using a partial filter in order to solve the above-mentioned problem, it is necessary to fix the position for disposing the partial filter on screen constitution.
It is already disclosed in FIG. 2 of Patent Literature 1 about an apparatus and a method for performing video observation of the arc welding.    Patent Literature 1: Specification of U.S. Pat. No. 5,255,088
A lens constitution and a system for filtering various lights generated under the arc welding environment are disclosed in Patent Literature 1. In the system according to Patent Literature 1, the contrast of the high-luminance light of the weld generated at the time of arc welding is reduced dramatically, and the suitable remote monitor is made possible. More specifically, the negative image of the arc welding part is formed by using a photochromic lens. In the system according to Patent Literature 1, the negative image of the arc welding unit formed using the photochromic lens functions as a density variable optical filter for reducing the luminance of the weld which illuminates on the photochromic lens and is formed an image. In the system according to Patent Literature 1, first of all, the light of the arc welding part is made to concentrate and form an image on the photochromic lens using a first lens, and next, the ultraviolet light is removed by using a second lens and the image input into a camera is formed.
In the system of above-mentioned Patent Literature 1 using the photochromic lens, there is a problem that a highly efficient PCF (Photochromic Filter) is needed, and also the light reducing performance is reduced rapidly when a focus shifts.
The purpose of the present invention is to provide a welding observation apparatus using partial darkening by a telecentric optical system and a PCF as an optical system for filtering light emission from arc discharge in order to verify the welding condition under the welding arc discharge, without forming an image on the PCF, so that light reducing performance can be secured even if a focus shifts from on the PCF, with satisfactory monitoring of welding condition.