{"id":4039,"date":"2024-09-30T10:00:09","date_gmt":"2024-09-30T14:00:09","guid":{"rendered":"https:\/\/designinglight.com\/?p=4039"},"modified":"2025-01-23T14:26:12","modified_gmt":"2025-01-23T19:26:12","slug":"how-are-cct-and-duv-calculated-ies-tm-40","status":"publish","type":"post","link":"https:\/\/designinglight.com\/?p=4039","title":{"rendered":"How are CCT and Duv Calculated? IES TM-40"},"content":{"rendered":"

If you look up the definition of Correlated Color Temperature (CCT) in IES LS-1<\/a> you’ll find, “The absolute temperature of a blackbody<\/a> whose chromaticity most nearly resembles that of the light source.” \u00a0It seems straightforward. \u00a0The spectra of non-incandescent light sources don’t exactly match a blackbody radiator. \u00a0They’ll plot off the blackbody locus in a chromaticity diagram. \u00a0A CCT calculation identifies the color temperature closest to the light source in question – that’s the CCT. \u00a0We all know that.<\/p>\n

Not long ago someone on the IES Color Committee suggested that we develop and issue a standardized CCT calculation. \u00a0My first thought was, “What? \u00a0Are you saying that after decades of specifying CCT there’s no industry standard calculation method? \u00a0What’s going on?” \u00a0It turns out that since the late 1930s at least a dozen CCT calculation methods have been developed but none of them have been adopted by a standards setting body like the IES or CIE. \u00a0 The same is true for Duv<\/a>, the direction and distance between the chromaticity coordinates of the light source and the nearest point on the blackbody locus. \u00a0This means that the method used is up to the LED or equipment manufacturer, and can vary from one manufacturer to another.<\/p>\n

Now, for lighting designers this isn’t a problem. \u00a0Variations between the calculation methods generally aren’t large (although they can range from less than 1\/1000th to several hundred K), and we are selecting LEDs described by their nominal CCT as outlined in NEMA C78.377, not their exact CCT. \u00a0Those chromaticity quadrangles are huge, roughly 400 – 500 K wide and over 7 MacAdam ellipses, as shown below. For lighting designers the problem is the huge variation possible within a single CCT designation, but that’s for another day.<\/p>\n

\"C78.377
NEMA C78.377 Chromaticity Quadrangles for LEDs<\/figcaption><\/figure>\n

But, for LED manufacturers, testing equipment manufacturers, and researchers this can be a real problem. \u00a0If two people attempt to precisely measure the same LED and arrive at two different CCTs how would they determine who’s right, or which calculation is “better”?<\/p>\n

Now we have an answer. \u00a0ANSI\/IES TM-40<\/a> IES Method for Determining Correlated Color Temperature (CCT) and Distance from the Planckian Locus of Light Sources<\/em> describes a CCT calculation method with an error of less than 0.1 K that calculates Duv based on the result of the CCT calculation. \u00a0While there’s no way to compel anyone to use this method, it is an American National Standard developed under the ANSI process, making it the closest thing we have to an industry standard. \u00a0Finally.<\/p>\n","protected":false},"excerpt":{"rendered":"

If you look up the definition of Correlated Color Temperature (CCT) in IES LS-1 you’ll find, “The absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.” \u00a0It seems straightforward. \u00a0The spectra of non-incandescent light sources don’t exactly match a blackbody radiator. \u00a0They’ll plot off the blackbody locus in a … Continue reading How are CCT and Duv Calculated? IES TM-40<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[12,21,11],"tags":[72,29,24],"class_list":["post-4039","post","type-post","status-publish","format-standard","hentry","category-color","category-education","category-leds","tag-color-science","tag-color-temperature","tag-led-color"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p4gZSw-139","jetpack_sharing_enabled":true,"jetpack_likes_enabled":true,"jetpack-related-posts":[{"id":1650,"url":"https:\/\/designinglight.com\/?p=1650","url_meta":{"origin":4039,"position":0},"title":"What is the Reference Illuminant?","author":"Jason Livingston","date":"May 11, 2021","format":false,"excerpt":"Over the past few months I\u2019ve had a manufacturer, a sales rep, and a lighting designer all tell me they think CRI compares a light source to daylight.\u00a0\u00a0When I tried to correct one of them the reply was an acknowledgment that an incandescent source is normally used, but daylight can\u2026","rel":"","context":"In \"color rendering\"","block_context":{"text":"color rendering","link":"https:\/\/designinglight.com\/?tag=color-rendering"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1264,"url":"https:\/\/designinglight.com\/?p=1264","url_meta":{"origin":4039,"position":1},"title":"Standard 189.1 Now Includes TM-30 Requirements","author":"Jason Livingston","date":"August 19, 2020","format":false,"excerpt":"Yesterday an addendum to ANSI\/ASHRAE\/ICC\/USGBC\/IES Standard 189.1-2017 Standard for the Design of High-Performance Green Buildings was published. The addendum makes changes to Section 8.3.5, which covers lighting. One of the biggest changes is to add TM-30 color rendition criteria to the section on Indoor Lighting Quality. Here's the relevant text:\u2026","rel":"","context":"In "Codes"","block_context":{"text":"Codes","link":"https:\/\/designinglight.com\/?cat=5"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":2434,"url":"https:\/\/designinglight.com\/?p=2434","url_meta":{"origin":4039,"position":2},"title":"Updating the CCT Calculation","author":"Jason Livingston","date":"March 8, 2022","format":false,"excerpt":"As I noted in Chapter 9 of the 2nd edition of Designing with Light, we calculate color temperature, correlated color temperature, and distance from the Plankian locus in a perverse way. \u00a0The calculations are performed in the CIE 1960 (u, v) chromaticity diagram (which is why distance from the Plankian\u2026","rel":"","context":"In "Calculations"","block_context":{"text":"Calculations","link":"https:\/\/designinglight.com\/?cat=6"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":787,"url":"https:\/\/designinglight.com\/?p=787","url_meta":{"origin":4039,"position":3},"title":"TM-30 Rf: So Big, So Strong, So Smart!","author":"Jason Livingston","date":"November 6, 2017","format":false,"excerpt":"As we know, CRI Ra and TM-30 Rf are both measurements of color fidelity. \u00a0That is, they compare a test light source to a known reference light source and measure how well the test source matches the reference source. \u00a0One of the many shortcomings of CRI Ra is that it\u2026","rel":"","context":"In "Color"","block_context":{"text":"Color","link":"https:\/\/designinglight.com\/?cat=12"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2017\/11\/CES-F32T8830.png?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2017\/11\/CES-F32T8830.png?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2017\/11\/CES-F32T8830.png?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2017\/11\/CES-F32T8830.png?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":1525,"url":"https:\/\/designinglight.com\/?p=1525","url_meta":{"origin":4039,"position":4},"title":"It’s Time for a Unified Chromaticity Diagram","author":"Jason Livingston","date":"March 1, 2021","format":false,"excerpt":"This post describes previous and current chromaticity diagrams for light and suggests the need for a new one.","rel":"","context":"In "Color"","block_context":{"text":"Color","link":"https:\/\/designinglight.com\/?cat=12"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2021\/03\/CIE1976uv.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2021\/03\/CIE1976uv.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/designinglight.com\/wp-content\/uploads\/2021\/03\/CIE1976uv.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":2910,"url":"https:\/\/designinglight.com\/?p=2910","url_meta":{"origin":4039,"position":5},"title":"CCT Doesn’t Predict Circadian Impact","author":"Jason Livingston","date":"January 2, 2023","format":false,"excerpt":"Two of my IES Color Committee friends and colleagues, Tony Esposito and Kevin Houser, have just published a paper in Scientific Reports that looks at the common assumption that CCT can be used to assess circadian entrainment and other biological impacts of light. The assumption by many is that high\u2026","rel":"","context":"In "Calculations"","block_context":{"text":"Calculations","link":"https:\/\/designinglight.com\/?cat=6"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"_links":{"self":[{"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/posts\/4039","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/designinglight.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4039"}],"version-history":[{"count":6,"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/posts\/4039\/revisions"}],"predecessor-version":[{"id":4334,"href":"https:\/\/designinglight.com\/index.php?rest_route=\/wp\/v2\/posts\/4039\/revisions\/4334"}],"wp:attachment":[{"href":"https:\/\/designinglight.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4039"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/designinglight.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4039"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/designinglight.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4039"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}