<\/p>\n
Details matter \u2013 High precision Raman<\/h2>\n
Raman analysis has been recognized to have the potential to identify the molecular make-up of many unknown different chemicals including drugs and pharmaceuticals (along with their counterfeits). \u00a0The signature Raman spectral \u201cfingerprints\u201d of chemical species can be observed using high precision, analytical laboratory tools such as Confocal Raman Microscopes, critical in research environments. However, many of the details that make this molecular species identification possible are associated with small energy exchanges between the incident and Raman scattered photons.\u00a0 As a consequence, access to this \u201cfingerprint\u201d region of the Raman spectrum requires detection of the Raman signals extraordinarily close to the laser line wavelength and thus requires extremely steep edge pass filters with accurate \u201ccutoff\u201d wavelength positions filtering.<\/p>\n
\u201cCutoff\u201d is defined as the spectral shift in wavenumbers (cm-1<\/sup>) between an optical density blocking of six orders of magnitude at the laser line wavelength (OD>6 or <0.0001% of the laser line power) and the 50% transmission point.\u00a0 Key spectral information in this fingerprint region can reside within a cutoff of 25-50 cm-1<\/sup>, defining the requirement for detection in the Raman instruments.<\/p>\n This creates several challenges for optical filter manufacturers.\u00a0 First a filter needs to be designed with a steep enough edge slope to achieve in theory this extremely low cut-off value.\u00a0 This can require filter designs with as many as 100s of tightly controlled individual dielectric layers or more.\u00a0 Secondly, even with a filter designed and manufactured with a steep enough edge slope, the wavelength targeting and uniformity across the part size (often 12.5mm up to 25mm) present challenges to control the manufacturing processes to this level of wavelength accuracy in a repeatable, manufacturable manner to realize this performance in practice.<\/p>\n <\/p>\n In contrast to trying to identify completely unknown materials from their chemistry or using Raman microscopy to study material structures, Raman\u2019s powerful ability to rapidly identify molecules non-invasively and without any sample preparation has created an explosion in demand for handheld\/portable Raman analytical devices intended to identify molecules from a smaller library of possibilities.\u00a0 These portable precision devices include small desktop instruments used in applications such as airport liquid screening, instruments with handheld probe attachments for dock-to-stock analysis of incoming goods such as pharmaceutical shipments in barrels, and handheld devices which can rapidly detect and identify illicit drugs, precursor chemicals, explosives, gemstones, raw materials, additives, residues of pesticide and veterinary drugs.<\/p>\n In order to provide the versatility needed in \u201creal-world\u201d environments this family of Raman instruments requires the ability to scan materials with a wider incident angle of light (AOI) and possibly an uncollimated cone half angle (CHA)), along with the ability to, as a consequence, accept higher wave number, increased cut-off values for the detected Raman signal (a wider passband wavelength range for the filter) such as >250 cm-1<\/sup> from the laser line.\u00a0 Extended operating ranges are often also required pushing the transmission band to longer wavelengths such as up to 1420nm or 1770nm (the lab units typically only operate between 300nm to 1200nm due to the Si detectors used with them).<\/p>\n These broad operating conditions create different filtering challenges than discussed above.\u00a0 While the filters are less complex in terms of layer count for the Raman edge-pass (due to the reduced steepness), they need to have controlled cut-off values to accommodate the broad angle ranges of use along with broad and deep blocking ranges. \u00a0Additionally, these filters are far more cost sensitive as the instruments using them can be an order of magnitude less expensive than their laboratory cousins and they need to be able to be integrated into the instrument passively (that is with less manual alignment) due to the volume nature of these builds.<\/p>\n <\/p>\n Clearly there need to be different optical filter solutions to address the needs of these to, related but very different market applications.<\/p>\n <\/p>\n [\/et_pb_text][et_pb_image src=”https:\/\/www.iridian.ca\/wp-content\/uploads\/2019\/11\/Edge-Pass-Filters-chart-1.png” align=”center” admin_label=”Chart 1″ _builder_version=”4.0.6″ transform_scale=”150%|150%” custom_margin=”||100px||false|false”][\/et_pb_image][et_pb_text admin_label=”Optical Filter Solutions:” _builder_version=”4.6.1″ header_2_text_color=”#0065cb” custom_margin=”||100px||false|false” z_index_tablet=”500″ text_text_shadow_horizontal_length_tablet=”0px” text_text_shadow_vertical_length_tablet=”0px” text_text_shadow_blur_strength_tablet=”1px” link_text_shadow_horizontal_length_tablet=”0px” link_text_shadow_vertical_length_tablet=”0px” link_text_shadow_blur_strength_tablet=”1px” ul_text_shadow_horizontal_length_tablet=”0px” ul_text_shadow_vertical_length_tablet=”0px” ul_text_shadow_blur_strength_tablet=”1px” ol_text_shadow_horizontal_length_tablet=”0px” ol_text_shadow_vertical_length_tablet=”0px” ol_text_shadow_blur_strength_tablet=”1px” quote_text_shadow_horizontal_length_tablet=”0px” quote_text_shadow_vertical_length_tablet=”0px” quote_text_shadow_blur_strength_tablet=”1px” header_text_shadow_horizontal_length_tablet=”0px” header_text_shadow_vertical_length_tablet=”0px” header_text_shadow_blur_strength_tablet=”1px” header_2_text_shadow_horizontal_length_tablet=”0px” header_2_text_shadow_vertical_length_tablet=”0px” header_2_text_shadow_blur_strength_tablet=”1px” header_3_text_shadow_horizontal_length_tablet=”0px” header_3_text_shadow_vertical_length_tablet=”0px” header_3_text_shadow_blur_strength_tablet=”1px” header_4_text_shadow_horizontal_length_tablet=”0px” header_4_text_shadow_vertical_length_tablet=”0px” header_4_text_shadow_blur_strength_tablet=”1px” header_5_text_shadow_horizontal_length_tablet=”0px” header_5_text_shadow_vertical_length_tablet=”0px” header_5_text_shadow_blur_strength_tablet=”1px” header_6_text_shadow_horizontal_length_tablet=”0px” header_6_text_shadow_vertical_length_tablet=”0px” header_6_text_shadow_blur_strength_tablet=”1px” box_shadow_horizontal_tablet=”0px” box_shadow_vertical_tablet=”0px” box_shadow_blur_tablet=”40px” box_shadow_spread_tablet=”0px”]<\/p>\n <\/p>\n To address the needs of the high precision Raman instruments, Iridian is able to offer a series of ultra-steep long pass edge and \u201cnano-edge\u201d long pass filters (LPFs). The ultra-steep (US) LPFs have cutoff values in the range of 40-50 cm-1 whereas the \u201cnano-edge\u201d LPFs have cutoff values of between 25-38 cm-1. Due to the steep nature of these steep LPF filters, the end user\u2019s system setup must have tight controls on AOI with a very well collimated input laser beam and well-known laser line wavelength in order to take optimal advantage of the low cutoff values that these filters are designed to provide.<\/p>\n [\/et_pb_text][et_pb_image src=”https:\/\/www.iridian.ca\/wp-content\/uploads\/2020\/10\/Chart-532nm-LPFs.png” title_text=”Chart – 532nm LPFs” align=”center” admin_label=”chart – 532nm LPFs” _builder_version=”4.6.1″ _module_preset=”default” width=”60%” module_alignment=”center”][\/et_pb_image][et_pb_text _builder_version=”4.6.1″ _module_preset=”default”]<\/p>\n Iridian\u2019s wide angle long pass filters (WA LPF) are a preferred solution for customers\u2019 instruments and devices with wider AOI ranges of 0-2 degrees, CHA up to 5 degree, and provide extended passband wavelength range with transmission > 93-95%, and extended blocking wavelength range with OD6 blocking over the broad operating ranges of these devices.<\/p>\n \u00a0LPF types<\/strong><\/p>\n<\/td>\n Laser Line [nm]<\/strong><\/p>\n<\/td>\n AOI Range [degree]<\/strong><\/p>\n<\/td>\n CHA [degree]<\/strong><\/p>\n<\/td>\n Cutoff [cm-1]<\/strong><\/p>\n<\/td>\n Blocking range [nm]<\/strong><\/p>\n<\/td>\n Pass Band Tx > 93% Range [nm]<\/strong><\/p>\n<\/td>\n Blocking (Optical Density)<\/strong><\/p>\n<\/td>\n<\/tr>\n Nano<\/p>\n<\/td>\n 785<\/p>\n<\/td>\n 0<\/p>\n<\/td>\n 0<\/p>\n<\/td>\n 26<\/p>\n<\/td>\n 785<\/p>\n<\/td>\n 789-1200<\/p>\n<\/td>\n OD 6<\/p>\n<\/td>\n<\/tr>\n US<\/p>\n<\/td>\n 785<\/p>\n<\/td>\n 0-1<\/p>\n<\/td>\n 0.1<\/p>\n<\/td>\n 40<\/p>\n<\/td>\n 785<\/p>\n<\/td>\n 790-1200<\/p>\n<\/td>\n OD 6<\/p>\n<\/td>\n<\/tr>\n WA<\/p>\n<\/td>\n 785<\/p>\n<\/td>\n 0-2<\/p>\n<\/td>\n 5<\/p>\n<\/td>\n 105<\/p>\n<\/td>\n 650-786<\/p>\n<\/td>\n 792.5-1200<\/p>\n<\/td>\n OD 6<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n Often dichroic filters are also used in the Raman probe and spectrometer systems where they are used to reflect the laser to the sample and also as first edge pass filter together with the second edge pass filter in the instrument. As a result, the dichroics used in these cases also need to cover the extended transmission bands for the receiving signals.<\/p>\n In both types of Raman systems, narrow laser line filters are employed to clean-up the excitation source and block any secondary peaks from reaching the sample under test.\u00a0 These narrow bandpass filters need to be designed to pass the laser line while ensuring that the blocking aligns with the transmission band of the chosen edge pass filter to avoid laser light leakage into the detection path.<\/p>\n While there are many off-the-shelf standard filter solutions available, it is important to ensure that the right filter is being chosen for the right task to optimize the technical performance and minimize the price for performance.\u00a0 Custom filter solutions (or tailored versions of standard solutions) can often be the best choice especially for OEM instruments where volumes will run into the many 10s to 100s of units. \u00a0By looking to optical filter vendors as partners or collaborators to help recommend and design the best solution for a given need these trade-offs and optimizations can be considered and designed in early, saving time and money and optimizing Raman signal output.<\/p>\n [\/et_pb_text][et_pb_button button_url=”\/rfq\/” url_new_window=”on” button_text=”Get a Custom Quote” button_alignment=”center” module_id=”popmake-14349″ module_class=”popmake-14349″ _builder_version=”4.6.1″ z_index_tablet=”500″ button_text_shadow_horizontal_length_tablet=”0px” button_text_shadow_vertical_length_tablet=”0px” button_text_shadow_blur_strength_tablet=”1px” box_shadow_horizontal_tablet=”0px” box_shadow_vertical_tablet=”0px” box_shadow_blur_tablet=”40px” box_shadow_spread_tablet=”0px”][\/et_pb_button][et_pb_text admin_label=”Page bottom Iridian tag line” _builder_version=”3.27.3″ text_font_size=”12px” text_orientation=”center” module_alignment=”center” z_index_tablet=”500″ text_text_shadow_horizontal_length_tablet=”0px” text_text_shadow_vertical_length_tablet=”0px” text_text_shadow_blur_strength_tablet=”1px” link_text_shadow_horizontal_length_tablet=”0px” link_text_shadow_vertical_length_tablet=”0px” link_text_shadow_blur_strength_tablet=”1px” ul_text_shadow_horizontal_length_tablet=”0px” ul_text_shadow_vertical_length_tablet=”0px” ul_text_shadow_blur_strength_tablet=”1px” ol_text_shadow_horizontal_length_tablet=”0px” ol_text_shadow_vertical_length_tablet=”0px” ol_text_shadow_blur_strength_tablet=”1px” quote_text_shadow_horizontal_length_tablet=”0px” quote_text_shadow_vertical_length_tablet=”0px” quote_text_shadow_blur_strength_tablet=”1px” header_text_shadow_horizontal_length_tablet=”0px” header_text_shadow_vertical_length_tablet=”0px” header_text_shadow_blur_strength_tablet=”1px” header_2_text_shadow_horizontal_length_tablet=”0px” header_2_text_shadow_vertical_length_tablet=”0px” header_2_text_shadow_blur_strength_tablet=”1px” header_3_text_shadow_horizontal_length_tablet=”0px” header_3_text_shadow_vertical_length_tablet=”0px” header_3_text_shadow_blur_strength_tablet=”1px” header_4_text_shadow_horizontal_length_tablet=”0px” header_4_text_shadow_vertical_length_tablet=”0px” header_4_text_shadow_blur_strength_tablet=”1px” header_5_text_shadow_horizontal_length_tablet=”0px” header_5_text_shadow_vertical_length_tablet=”0px” header_5_text_shadow_blur_strength_tablet=”1px” header_6_text_shadow_horizontal_length_tablet=”0px” header_6_text_shadow_vertical_length_tablet=”0px” header_6_text_shadow_blur_strength_tablet=”1px” box_shadow_horizontal_tablet=”0px” box_shadow_vertical_tablet=”0px” box_shadow_blur_tablet=”40px” box_shadow_spread_tablet=”0px” global_module=”14031″ saved_tabs=”all”]<\/p>\n Iridian Spectral Technologies Ltd<\/span> <\/strong>(Iridian) is a diversified optical filter manufacturer that is an international leader in filter design and manufacture especially for application in the fields of fiber optic communications, optical spectroscopy, 3D entertainment, and in aerospace. Iridian is a global supplier with distributors in many countries.<\/p>\n [\/et_pb_text][\/et_pb_column][\/et_pb_row][\/et_pb_section]<\/p>\n","protected":false},"excerpt":{"rendered":" Raman spectroscopy is a powerful and increasingly ubiquitous analytical tool capable of identifying molecular constituents of samples under test and, when combined with microscopy, exploring specific cellular structures and functions. Non-invasive, non-contact, requiring no sample preparation or chemical tagging \u2013 it is no wonder that Raman has established a presence as an invaluable analytical technique both in labs and in the field. <\/p>\n","protected":false},"author":159,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_et_pb_use_builder":"on","_et_pb_old_content":" Raman spectroscopy probes the molecular vibrational and rotational modes of a material in order to detect and identify the material.\u00a0 Typically, laser light is incident upon the material and the scattered light is measured.<\/p> The excitation source (laser line) intensity is often \u00a0to \u00a0orders of magnitude greater than the Raman scattered signal. Therefore, edge pass (or notch) filters are required to block the Rayleigh scattered laser light while transmitting the red-wavelength shifted (Stokes) and\/or the blue-wavelength shifted (Anti-Stokes) Raman scattered signal.<\/p> This technical note first defines various terms useful in describing Raman edge (or notch) filters.\u00a0 After this, there is a discussion on how to best use the edge pass filter to ensure that the desired optical performance is obtained for the intended application. \u00a0Different parameters that affect the measured edge pass filter transmittance are described including angle of incidence, light cone incident upon the filter, polarization of light and laser line wavelength variation.<\/p> Optical Density<\/u> (OD) is a measure of the blocking ability of an optical filter. \u00a0As light passes through the filter, some of the light is transmitted while the rest is either reflected, scattered or absorbed. Optical density takes into account all forms of light attenuation and is defined as:<\/p> OD = -log10<\/sub>T<\/strong><\/p> where \u2018T<\/em><\/strong>\u2019 is the transmittance of a filter.\u00a0 The OD value of a filter is numerically equivalent to \u00a0of the filter\u2019s transmittance when specified in dB.<\/p> Examples of different OD levels versus T[dB] for a Long-Pass Filter (LPF) are shown in the figure below.\u00a0\u00a0 Unless otherwise stated, this and the following figures assume collimated, monochromatic light incident upon the filter with a negligible instrument (detector) resolution.<\/p>[caption id=\"attachment_14287\" align=\"aligncenter\" width=\"864\"] \u00a0<\/p> The edge steepness<\/u> of an edge pass filter is defined as the spectral width (specified in units of \u2018%\u2019, \u2018nm\u2019 or \u2018cm\u20111<\/sup>\u2019) between two points on the slope of the edge pass filter. \u00a0Hence, the smaller the edge steepness of a filter, the sharper the transition from passing light to blocking light (and the more challenging it is to fabricate the filter in order to maintain a low passband ripple).\u00a0 Note that the \u2018spectral edge<\/u>\u2019 of an edge pass filter is typically defined as the wavelength where the filter transmittance = -3 dB (50%).<\/p> \u00a0<\/p> In the figure below, the edge steepness is defined (typically) as the spectral width between the -3dB (50%) point of a filter and the -60 dB (6 OD) point of a filter, where the latter is usually the minimum allowed OD of the filter at the laser line wavelength. \u00a0Note that the edge steepness could also be defined differently depending on the filter application (i.e.,\u00a0 spectral width between 90% and -60 dB of a filter).<\/p> \u00a0<\/p>[caption id=\"attachment_14285\" align=\"aligncenter\" width=\"870\"]Is it this or is it that? \u2013 Handheld \u201cin-field\u201d Raman<\/h2>\n
Optical Filter Solutions:<\/h2>\n
\n\n
\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n Raman Spectroscopy<\/h2>
![\"\"](\"https:\/\/www.iridian.ca\/wp-content\/uploads\/2019\/11\/Edge-Pass-Filters-chart-1.png\")
<\/a><\/p>\u00a0<\/h2>
Optical Density<\/h2>
<\/a> Example of a typical Long Pass Filter (LPF) shown with the filter transmittance in dB and linear scales. Different OD levels corresponding to T[dB] are also shown.[\/caption]\u00a0<\/h2>
Edge Steepness<\/h2>
<\/a> Figure showing how \u2018Edge Steepness\u2019 of a LPF is typically defined (in this case, between \u00ac 3dB and -60 dB transmittance levels).[\/caption]\u00a0<\/h2>
\u00a0<\/h2>
Cut-Off \/ \u201cOD Blocking at the Laser Line Wavelength\u201d<\/h2>